Published: March 16th, 2019
The efficacy of public health pesticides targeting nuisance and disease-vector insects is not uniform across different ecological zones. Here we present a system of techniques using captive vector insects as sentinels for pesticide efficacy to derive electronic maps supporting the standard evaluation of pesticides across multiple environments.
Efficacy of public health pesticides targeting nuisance and disease-vector insects such as mosquitoes, sand flies, and filth-breeding flies is not uniform across ecological zones. To best protect public and veterinary health from these insects, the environmental limitations of pesticides need to be investigated to inform effective use of the most appropriate pesticide formulations and techniques. We have developed a research program to evaluate combinations of pesticides, pesticide application equipment, and application techniques in hot-arid desert, hot-humid tropical, warm and cool temperate, and urban locations to derive pesticide use guidelines specific to target insect and environment. To these ends we designed a system of protocols to support efficient, cost-effective, portable, and standardized evaluation of a diverse range of pesticides and equipment across multiple environments. At the core of these protocols is the use of an array of small cages with colony-reared sentinel mosquitoes (adults and immatures) and sand flies (adults), strategically arranged in natural habitats and exposed to pesticide spray. Spatial and temporal patterns of pesticide efficacy are derived from percent mortality in sentinel cages, then mapped and visualized in a geographic information system. Maps of sentinel mortality data may be statistically compared to evaluate relative efficacy of a pesticide across multiple environments, or to study multiple pesticides in a single environment. Protocols may be modified to accommodate a variety of scenarios, including, for example, the vertical orientation of sentinels in canopy habitats or simultaneous testing of ground and aerial application methods.
Efficacy of public health pesticides targeting nuisance and disease-vector insects such as mosquitoes, sand flies, and filth-breeding flies is not uniform across desert, tropical, temperate, or urban ecological zones1. Certain key species in these three groups of insects are important vectors of parasites, viruses, filarial worms, and bacteria that cause significant diseases in humans, pets, and livestock worldwide. To best protect public and veterinary health, the environmental limitations of pesticides must be investigated to inform effective use of the most appropriate pesticide formulations and techniques. Public health pesticide manufacturers are not required by the U.S. Environmental Protection Agency to specify the expected efficacy of a formulation across a range of environments or target insects, yet these pesticides are used for mosquito and vector control across multiple ecological zones in the U.S. and around the world.
We have developed a research program to evaluate numerous combinations of pesticides and pesticide application equipment and techniques in hot-arid desert, hot-humid tropical, warm and cool temperate, and urban locations, in order to derive pesticide use guidelines specific to the target insect and environment1. In this program, we evaluate pesticides that target adult stages of mosquitoes and sand flies (adulticides) and immature stages of mosquitoes (larvicides) using pesticide application equipment that is hand carried, truck- or aircraft-mounted, and installed in fixed locations. Then, four major outdoor pesticide application techniques are evaluated: (1) ultra-low volume (ULV) or thermal fog aerosol space sprays of adulticides designed for rapid knockdown of target insects, (2) a variant of the first technique in which liquid larvicides are applied with ULV or thermal fog for short- or long-term suppression of immature stages of target insects, (3) timed misting sprays from fixed locations designed to repel or kill, and (4) low volume (LV) cold mist sprays of residual pesticides designed to apply long-lasting toxic or repellent coatings on a variety of natural or artificial substrates. Presented here are the detailed methods for conducting techniques (1) and (2) mentioned above. Methods for (3) will be presented in separate studies, and techniques in (4) are described in brief in earlier publications2,3,4.
To accomplish this complex research program, we designed a system of protocols to support efficient, cost-effective, portable, and repeatable/standardized evaluation of aerosol adulticide and larvicide techniques with diverse pesticides/equipment combinations across multiple environments. At the core of these protocols is the use of colony-reared sentinel mosquitoes (adults and immatures) and sand flies (adults) to indicate spatial and temporal patterns of pesticide efficacy. In the case of adulticide applications, sentinel adult mosquitoes or sand flies are contained in small single-use disposable cages distributed in structured arrays through the target area and an untreated control area. For larvicide applications, small single-use disposable cups are similarly distributed to collect sprayed larvicide droplets for later introduction of water and sentinel colony-reared immature mosquitoes. Next, we record percent mortality in sentinel cages, or percent adult development in sentinel cups, at set intervals post-spray and use these data to produce electronic maps of spatial and temporal efficacy in a geographic information system (GIS) that may be quantitatively compared between and among environments.
Using sentinel cages of colony reared insects to evaluate pesticide efficacy in the field is a long-established practice5,6, and using empty plastic sentinel cups for collecting sprayed larvicide is emerging in the literature7. However, our electronic mapping of efficacy to visualize spatial and temporal patterns of mortality is an innovation that greatly improves investigation of mortality otherwise presented in flat table formats. Also, the high throughput cage loading system and the modular cage deployment system adaptable to diverse scenarios described here are unique to our program. Other research programs approach evaluation of pesticide applications in the field differently. Current popular methods include capturing and analyzing dye-labeled pesticide droplets from sprays in the field on spinning glass slides8 or acrylic rods9, which is a long-established process producing data that can be electronically mapped and visualized.
One drawback is that droplet size and density measurements from collection media are only estimated from a small proportion of the total collection surface, with software-assisted microscopy of fields of view that are unfortunately highly subjective. Also, maps of droplet distribution and density do not fully illustrate pesticide efficacy, because the assumption is that the presence of a threshold number of droplets of a certain size automatically indicates target insect mortality. This assumption does not account for mortality from evaporative products from droplets through the target area that may also induce mortality10, or that a lower number of droplets or other droplet sizes may kill some proportion of target individuals. The original rationale10,11,12 is that an aerosol pesticide is designed to impinge small droplets on actively flying target insects. However, our observations in the field, including reductions in natural populations after spraying when target insects are not actively flying, suggest that droplets or evaporative products from droplets are reaching targets that are not flying but rather hidden in resting refugia (unpublished data 2011). Also, we have observed in analysis of a field spray application (via simultaneous droplet capture, pesticide active ingredient capture, and sentinel cages) that maps of droplet distribution, active ingredient distribution, and mortality are not concordant (unpublished data 2010).
Another popular approach to evaluating pesticide efficacy is deploying sentinel cages in a grid delineated in a flat mowed homogeneous field with no obstructions to the pesticide plume, and under near ideal meteorological conditions (e.g., consistent winds <10 mph and wind direction perpendicular to the spray line). Still, others approach this by measuring efficacy with sentinels placed in wind tunnels14. These approaches provide one perspective on pesticide efficacy, but are less likely to realize operational efficacy under non-ideal field conditions (heterogeneous habitats that include obstructions to pesticide flow and variable if not under sub-optimal meteorological conditions). Seeking evidence in support of absolute efficacy is not realistic. Operational conditions are rarely ideal, and choosing formulations based on tests in wind tunnels or direct application in engineered parks may be misleading.
In our research, we use natural field sites and the given meteorological conditions (but not in rain or extreme winds that are outside the limits of any operational program). This is likely more informative for operational vector control when observing reasonable efficacy in a pesticide formulation despite poor environmental conditions, heterogeneous habitat, and obstructions to pesticide flow. Whenever possible, it is recommended to supplement sentinel insect mortality data with before- and after-surveillance of natural target insect populations in the treatment and control areas, as a bridge between controlled exposure and realized exposure to the focal pesticide. However, surveillance of natural populations is not sufficient to determine whether pesticide application produces mortality in a target population or whether target insects actually move from the target area after detecting oncoming pesticide aerosols.
Regardless of the caveats to any evaluation of pesticide sprays in the field, electronic mapping of mortality data in a GIS (as opposed to flat presentation of mortality data in tables) retains the quantitative attributes capable of rigorous comparison across trials and also provides a means for quick, visual assessment. With data captured in the GIS, researchers can set thresholds for pesticide efficacy and visualize the relative capability of a focal pesticide across multiple environments, or they can compare capabilities of multiple pesticides within a single environment across a variety of application equipment and techniques.
Note: This protocol is written specifically for field trials targeting adult mosquitoes. Information on modifications necessary for immature mosquitoes, other adult sentinel insect species, and unique scenarios is included in the discussion.
1. Sentinel Insect Rearing and Sentinel Cage Preparations
Figure 1: Sentinel cage preparation. (A) Shown here are two personnel loading sentinel cages with anaesthetized mosquitoes spread on a large white sheet of paper. The stack of tulle mesh squares in the foreground ready for placement on the waiting open sentinel cages should be noted. (B) Shown are several loaded sentinel cages, awaiting placement of 10% sucrose cotton balls and realignment of the rubber band. (C) Shown are sentinel cages in a cooler, ready for deployment in the field. Please click here to view a larger version of this figure.
2. Preparation of the Field Site
Figure 2: Prepare field site with sentinel cage poles. Two scenarios of sentinel cage poles distributed in (A) irrigated tropical microhabitat in a hot-arid region and (B) hot-arid desert. In (A), three kinds of sentinel apparatus are shown: the "ladder" on the left is a series of cotton ribbons suspended between PVC pipe to capture pesticide at various heights above ground; the pole in the middle is for sentinel mosquito cages; and the pole on the right supports a slide spinner to capture pesticide droplets. In (B) there is a similar cotton ribbon to capture pesticide, but it should be noted that the right-hand support of the apparatus in the foreground in the open doubles as a sentinel cage pole (the sentinel cage attached about one-third of the way down); in contrast, the apparatus in the background is placed so that the ribbon and the sentinel cage (indicated with yellow arrow) are sheltered within vegetation. The aircraft in the background is conducting a ULV pesticide spray. Please click here to view a larger version of this figure.
3. Deploying the Sentinel Cages
Figure 3: Placement of sentinel cages indoors and outdoors. Examples of pole placements and 1 ft3 boxes (A) indoors in a simulated rural residence, (B) outdoors in a simulated neighborhood, and (C) in a date palm grove in a hot-arid zone. Also shown are the sticky tiles under the box and under the sentinel cage placed in the open in (A), and the sentinel cage pole pin pushed through the sticky tile next to the box in (B) to reduce incursion of ants into sentinel cages. The plywood square with upright PVC mount to the left of the box in (A) is a support for a slide spinner. A similar apparatus may be used to support a sentinel cage pole to allow placement of sentinels at varying heights. Also shown is the slide spinner with slides on a mount to the left of the sentinel cage pole and box (B). In (C) is shown a high-low placement of two sentinel cages to investigate movement of the pesticide at different levels. The top of the opened sentinel cage cooler is just visible in the foreground. Please click here to view a larger version of this figure.
Note: Depending on size of the grid, ensure there are sufficient personnel to deploy all cages in an appropriate time frame to minimize environmental effects on sentinel mortality. It may be necessary to conduct practice runs with empty cages to determine whether enough personnel are present. It is understood that there will unavoidably be some spread in the exposure time of sentinel cages to the environment, owing to set-up time with the given number of cages per person. In very large sentinel grids (e.g., for some aerial applications), plan on coordinating multiple teams in several vehicles to place the sentinel cages within a reasonable time frame. It is inevitable that some mortality will occur across the sentinel cage mosquito population, so it must be accounted for with careful cage labeling prior to environmental exposure outside the coolers and prior to pesticide application.
4. Conducting the Pesticide Application
Figure 4: Vehicle mounted thermal fog spraying a grid in warm-arid equatorial location. A truck mounted ULV sprayer driving along a spray line directing pesticide spray through a grid of sentinel cages in an open field (A). Similar to Figure 2, each sentinel position is fitted with a second pole to support a cotton ribbon to collect pesticide droplets for later analysis with gas chromatograph/mass spectrometry. Close-ups of sentinel cages (B) in a warm temperate environment before spray and (C) in a hot-arid environment after spray. Please click here to view a larger version of this figure.
5. Collecting the Sentinel Cages and Recording Mortality Data
Figure 5: Processing sentinel cages post-spray. Two scenarios of transferring post-spray cages into stacked trays during the 6 h mortality check: (A) in a hotel room near a remote field site and (B) in a convenient parking lot returning to the lab from a distant field site. Note the PVC spacers in the inset photo in (A), moist towels covering cages laid out in trays in the main photo of (A), and an example of a unique location code and pre- and circled post-spray mortality annotated directly on a sentinel cage in the inset photo in (B). Please click here to view a larger version of this figure.
6. Processing, Analyzing, and Mapping Mortality Data
Here are representative results presented from two unpublished field studies that included the core of the methods described above. In these studies, two aspects of adulticide efficacy against sentinel disease vector insects were investigated.
The first study (unpublished data 2010-2012) investigated whether diluent might influence efficacy of pesticide against mosquitoes in a hot-arid desert environment applied with a thermal fog device. We conducted three separate applications with a synergized permethrin adulticide capable of being diluted in either oil or water. Each of the applications was conducted using a truck-mounted thermal fog generator with a different diluent: either water, BVA13 mineral oil, or diesel. Then, a grid of at least 20 sentinel mosquito cages placed on poles in an open arid-land area (Figure 6) was used and meteorology was recorded using a portable weather recorder.
The second study (unpublished data 2011) investigated the relative efficacy of a single pesticide formulation applied simultaneously with two kinds of sprayers (ULV and thermal fog) in a hot equatorial environment against sand flies. We used two adjacent grids of 25 sentinel sand fly cages placed on poles in low grass-forb habitat of varying density in a large field in a hot equatorial valley basin (Figure 7). Meteorology was recorded using a portable weather recorder positioned between the two grids. One truck carried the ULV device and another carried the thermal fog generator, and both initiated sprays along the spray lines at the same time, each moving from east to west at a speed appropriate to the label-specified application rate and given flow rate of the sprayer.
In both field studies, after a 10-min hold time post-spray, we collected all sentinel cages from treatment and control areas, simultaneously initiating the process of recording post-spray mortality. Percent mortality data was then coded, corrected for observed background control sentinel mortality, and placed into a GIS coverage consisting of georeferenced points corresponding to the locations of the sentinel cages in the treatment area grid (Figure 6 and Figure 7). Despite the ideal situation in which a pesticide spray produces a 100% kill throughout the target area, the realistic threshold for acceptable efficacy of a pesticide is arbitrary. Expectations of efficacy could vary with distance from the sprayer (e.g., setting a threshold of 95% mortality 50 ft from the spray line, with 80% mortality at 250 ft).
The results from both representative studies naturally show the spectrum of positive to negative outcomes, because the color ramp represents areas of 0 to 100% mortality (see color ramps in Figure 6 and Figure 7). All mortality data in the treatment area are normalized by the background mortality in the untreated control area to a threshold of 25% control mortality18, above which would have discarded due to excessive environmental or colony effects on mortality. The utility of the electronic mapping approach for visualizing mortality data is evident here: the researcher and (later) the reader can instantly understand the relative efficacy of the focal pesticides and diluents (Figure 6) or the focal pesticide sprayers (Figure 7) against mosquitoes or sand flies, respectively. It is valuable to compare Figure 6 to the underlying mortality data that would traditionally be presented in tabular form (Table 2) and require much more internal conceptualization, with commensurate increased potential for error, by researcher and reader alike. The electronic map, if supplemented with meteorological data and a satellite photo base coverage, also facilitates rapid assessment of potential effects of the habitat on the pesticide applications. It should be noted that the inset wind rose diagram in Figure 7 has a wind angle that perfectly matches the angle of spatial mortality in the west grid, indicating that the spray truck should have started further to the east so that pesticide would have an opportunity to reach all sentinel cages. If mortality data is only been considered in tabular form, it may be easy to miss this weakness in the experimental design and assign a lower overall efficacy to that pesticide application equipment, biased by zero mortality values from areas not contacted by the pesticide.
In our experience, the majority of pesticide applications in the field produce a gradient of mortality through the target area sentinel cages, which automatically demonstrates that the application was valid. However, if zero mortality is observed throughout the treatment area and the spray event is valid (i.e., the spray cloud moving through the target sentinel area is observed), it can be inferred that the pesticide is not effective with that species at the rate applied, in that environment, and with that application equipment. Of course, this is given that the pesticide batch is not expired nor has been stored inappropriately. On the other hand, some aerial pesticide applications in particular may produce no visible or detectable spray cloud impacting the target area, and zero mortality throughout sentinels may mean that the spray missed the target area. It is advised to anticipate this scenario by setting up a series of extra sentinel cages that brackets the target impact area to some distance upwind and downwind (e.g., 50 ft intervals for at least one swath width in each direction) so that some indication of touchdown of the pesticide may be gleaned if the target area is missed.
Figure 6: Representative interpolated sentinel cage mortality data from a field trial in hot-arid desert conditions targeting mosquitoes. In this series of sprays, the thermal fog pesticide sprayer, pesticide, and environment were kept constant, varying among pesticide diluent to (A) water, (B) diesel, and (C) BVA 13 mineral oil across the three trials. Please click here to view a larger version of this figure.
Figure 7: Representative interpolated sentinel cage mortality data from a field trial in hot-equatorial conditions targeting sand flies. In this set of two simultaneous spray applications the pesticide and environment were kept constant, but the pesticide sprayer was different between the two grids: thermal fog (west grid) and ultra-low volume (ULV, east grid). Note that the wind direction indicated in the wind rose diagram perfectly matches the angle of spatial mortality in the west grid, indicating that the spray truck should have started further to the east so that pesticide would have an opportunity to reach all sentinel cages. Please click here to view a larger version of this figure.
|DATE OF APPLICATION:
|SENTINEL INSECT SPECIES:
|LIFE STAGE OF SENTINELS:
|[copied directly from sentinel cages; data observed in field]
|[observed from sentinel cages stored in trays]
|Sentinel Cage Code
|No. DEAD POST-SPRAY/HOLD TIME
|No. DEAD 6 hr POST-SPRAY
|No. DEAD 12 hr POST-SPRAY
|No. DEAD 24 hr POST-SPRAY
Table 1: Sample mortality data form. The form has spaces for general information about the field trial in the top left, which is critical to manage data from multiple field projects. The main section of the form has spaces for pre- and post-spray mortality (both copied directly from the sentinel cages), through the later 6 h, 12 h, and 24 h checks. Handwritten data on this form are entered into a similar electronic spreadsheet, with added columns to correct mortality data for pre-spray dead and to correct spray area mortality for any environmentally-induced mortality in the control area.
|PERCENT MORTALITY (ABBOTT-CORRECTED)
|Sentinel Cage Code
|(A) Aqualuer + Water
|(B) Aqualuer + Diesel
|(C) Aqualuer + BVA13
Table 2: Underlying mortality data used to create the interpolated efficacy map in Figure 6. It should be noted that some spatial information can be captured in this table; for instance, this can be done by separating rows and columns by distance from sprayer and distance along spray line, respectively. However, temporal changes in mortality corresponding to immediate post-spray, 4 h post-spray, and 12 h post-spray, for example, would require a more complex table or additional tables. Similarly, separate tables for each diluent used in the trials are necessary for clarity if the tables are partitioned by space and time.
Combining the classic sentinel cage approach with electronic mapping of interpolated mortality data is a unique and powerful method to evaluate pesticides in the field, and it supports comparative pesticide efficacy studies across multiple environments and diverse configurations of pesticides and application techniques. Although the basic sentinel cage method is not new, the visualization of sentinel cage mortality patterns in a GIS is an advancement conducive to deeper analysis of patterns of flow of aerosol pesticide sprays. The interpolation of point measurements of pesticide efficacy into a color-coded map coverage is similar to adding smoke effusions to a wind tunnel to visualize air flow around an automobile, and it is a major improvement for reporting spatial and temporal mortality data in a series of tables.
Some parts of pesticide sprays can be seen with the naked eye, and pesticide droplets from invisible portions of sprays can be captured on glass slides or other media which, like sentinel cages, are long-established protocols. Evaporative products from droplets and droplets themselves can be captured with cotton ribbons and analyzed with a gas chromatograph/mass spectrometer, which provides even more information about the fate of a pesticide spray. However, the realized efficacy of the spray and spatial patterns of actual pesticide-induced mortality in the target area (which may include both droplet and evaporative product components) can only be definitively measured by sentinel insects.
Furthermore, the temporal component of efficacy can only be definitively measured by a series of mortality observations in sentinel insects that capture quantified indices of rapid knockdown versus long-term morbidity and mortality on the target species. Again, a series of interpolated color-coded maps can be used to clearly visualize the evolution of post-spray mortality over time, with an explicit spatial component, in a way that a series of tables is not able to communicate to a reader. The series of maps can be animated in a loop to reproduce the progress of the spray and its effects on sentinel insects, further enhancing the understanding of efficacy not only for a single trial, but in comparisons among pesticides, techniques, application equipment, target insects, and ecological zones.
For the highest quality mortality data in this method, great care should be taken throughout the start-to-finish handling and observations of the sentinel insects. Exposure time and conditions of sentinel insects to the environment across both the treatment and control areas before the pesticide spray should be as equal as practical. This exposure should include a period of acclimation to the ambient conditions and a uniform hold time post-spray for both treatment and control insects. Observations of mortality before the spray, after the hold time (i.e., during retrieval of cages), and for the designated periods post-spray should be carefully tracked on data forms. Care should also be taken to retrieve all sentinels from the field before departing for the lab. Control and treatment sentinel cages should be physically separated throughout the protocol. Reliable observations of background baseline mortality from the control cages at all designated time periods are critical to appropriate correction of observed mortality in the treatment zone. Accurate, precise, comparable, and meaningful efficacy maps can only be gained from high-quality mortality data input to the geographic information system.
The sentinel insect method is naturally flexible to be relevant in a variety of scenarios - anywhere a small cage can be placed, mortality data can be collected. For example, we have conducted pesticide trials with sentinel cages placed in and around simulated urban and rural buildings10 and U.S. military tents (unpublished data 2017-2018), in addition to multiple scenarios of desert, temperate, and tropical vegetation19,20,21,22 [including hoisting cages up to 60 ft into pine canopy to measure vertical mortality following a large aerial application (unpublished data 2011-2017)]. If the ground is too hard to place sentinel cage poles or it is preferred to place them on concrete or asphalt areas, simple stands or concrete blocks can be constructed to support the poles. For scenarios to investigate sprayed liquid larvicides, the protocol can be modified to place empty plastic disposable 1 qt cups to capture larvicide across sentinel locations. These cups can be later filled with water and mosquito larvae to measure efficacy of the application7,23,24,25. Use floor tiles with the sticky side up to keep cups in place in wind and keep lids nearby to rapidly cap and collect following the post-spray hold time. Alternatively, cups can be left in place to weather naturally or left open in a controlled environment to investigate the longevity of a residual larvicide treatment.
To investigate droplet density and droplet spectra throughout the application area, slide spinners can be placed near sentinel insect positions - though exercise caution that the vortex from the spinning slides does not affect the flow of the pesticide spray to the sentinel insects. Similar to mapping mortality, additional columns in the attribute table for the sentinel locations can be added for droplet and dye parameters to derive interpolated coverages. Note that adding droplet collection aspects will demand an increase in the number of personnel in the field, with dedicated teams for example to carefully collect slides and assist the spray operator with dye additives. With additional materials and teams of personnel, these methods may be merged to conduct simultaneous trials using larval and adult sentinels, multiple application modes (aerial, ground, portable), or pesticides side-by-side (see representative results).
Although the main protocol was written for mosquitoes, we have successfully conducted field trials with sand flies and filth-breeding flies as sentinels with only minor modifications to the sentinel cages and overall protocol. For example, it is not practical to sex adult sand flies or filth breeding flies so that mixed-sex batches are used in sentinel cages, as this will reduce load on the colony because fewer specimens are needed than when working with mosquitoes. For sand flies, a very fine mesh must be used for the sentinel cages; furthermore, sand flies are not anaesthetized but instead added with aspirators directly into fully assembled cages through a rubber slit glued over a hole cut in the side of the cylinder.
Figure 8: Additional scenarios demonstrating the flexibility of the sentinel system. The sentinel protocol for investigating pesticide efficacy in the field is very flexible, as shown in sentinel cages hoisted at intervals up to 60 ft through pine canopy (A) and a nearby open area (B) to investigate capability of aerial ULV pesticide spray to penetrate canopy. The sentinel system can easily be adjusted to examine larvicide sprays targeting immature mosquito stages using disposable plastic cups to capture droplets (C) indoors and (D) outdoors in a simulated urban area. Please click here to view a larger version of this figure.
The efficacy mapping method is naturally flexible because it is based on interpolation which is a standard process in most GIS programs26. Generally, interpolation uses known data at set points to estimate data at nearby unsampled points. There exist several types of interpolation techniques27 which may be selected based on the spatial spread and density of the point mortality data. We have used inverse distance weighting (IDW), which assigns higher statistical weight to known data from points closer to the unknown points being estimated. Troubleshooting for the field portion of the method is centered on control sentinel mortality (i.e., if mortality is > 25% in controls, and there is certainty that the application did not impact the control area, something in the environment other than the pesticide is causing mortality, which will confound analysis; then, the trial will need to be repeated or moved to another location). The most common vulnerabilities in the mapping portion of the method are production of quality data tables in the GIS, so it is critical to carefully paste data so that the right data are aligned with the right points (reading frame) and correctly label columns for each mortality time period, pesticide, application equipment, etc.
The sentinel insect method is not designed to be an absolute measure of efficacy. Rather, the method provides the ability to compare relative efficacy of a pesticide (under a given environment, application equipment, diluent, target insect, and technique) to the same pesticide under different conditions, or to compare different pesticides under the same conditions. The method does not include pesticide droplet or active ingredient capture, though apparatuses for investigating these aspects can easily be placed in the grid adjacent to sentinel cages. The sentinel cage method does not measure efficacy of pesticide sprays against vector insects in flight, which is possible but not practical28. A lively controversy exists whether the kind of mesh on sentinel cages effects measurements of pesticide spray efficacy14,29,30,31. However, this is not hugely relevant to our objective of investigating relative efficacy of a formulation across environments or several formulations within an environment (which can be conducted using a standard cage with a standard mesh type).
For example, a recent study compared sentinel cage mortality results across three similar aerial pesticide application experiments separated by decades, each using different sentinel cage methods, all with comparable results10. Similarly, controversy exists over whether to transfer sentinel insects to "clean" cages (i.e., cages that have not been sprayed). Again, data from sentinel insect mortality should be considered relative and not absolute and are not concerned with additional mortality resulting from sentinel insects contacting pesticide adhering to the cage or mesh. In fact, in natural environments, pesticide sprays will also adhere to natural surfaces that target insects may contact. We have previously found that mortality induced from handling mosquitoes, to include CO2- or cold-based anaesthetization, may exceed mortality from contacting a pesticide that could have been present on cages (unpublished data 2008). Another limitation is that the relevance and applicability of sentinel cage mortality results in the local natural populations of the target insect still not being fully known; however, the closer genealogically the colony-reared sentinel insects are to the local species, the stronger the applicability of the efficacy maps is to local populations.
In future variations of this method, it would be beneficial to include modifications to accommodate pesticide applications with unmanned aerial systems (UAS). Current developments with UAS use in operational vector control include pesticide applications (in particular, larvicide formulations targeted at immature mosquito habitat) over and through highly inaccessible locations. To gain relevant information from sentinel trials, sentinel insect stations would need to be placed throughout inaccessible locations to the greatest extent possible. An example of relevant information is testing the capability of UAS effectively reaching a certain area with larvicide, with the only line-of-sight piloting by an operator who cannot directly observe the target area. This scenario may require development of other UAS to deploy and retrieve sentinel cages or larvicide collection cups and others to record meteorology in these inaccessible target areas. Mortality data from such a scenario can be analyzed in the GIS as with established scenarios, with added map features such as effects of natural obstacles, microhabitat, and micrometeorology that may differ from such effects during more standard applications with truck, aircraft, or portable sprayers. Advanced capabilities of GIS such as visualization of 3-dimensional interpolations of mortality from enhanced sentinel placement in vertical and horizontal grids are also possibilities for both standard and emerging application technology.
The authors have no conflicts of interest to disclose.
We would like to thank the scientists and technicians at the Coachella Valley Mosquito and Vector Control District and the U.S. Army Medical Research Directorate-Kenya for expert production of colony insect specimens and collaboration on the field studies producing the unpublished data presented in the representative results. This research was supported by the U.S. Department of Agriculture (USDA)-Agricultural Research Service and the U.S. Department of Defense (DoD) Deployed War-Fighter Protection Program (DWFP). Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA, DoD, or DWFP. The USDA is an equal opportunity provider and employer.
|plastic tube lid
|1.25-in PVC coupler SCH-40
|PVC 00100 0800
|1/4-in OD brass rod
|FDA silicone o-rings S-500-70
|Alltek seal and packing
|Large cotton balls (non-sterile)
|Wika Instrument Corp
|20 lb capacity
|Modified tupperware container (16 cup)
|1/4-in tygon tubing
|maglite aspirator and tubes
|D-cell maglite aspirator
|modified PVC pipe for o-rings
|PVC 07112 0600
|SCH-40 pipe modified by cutting tool on inner surface to accommodate bioassay tube
|John W. Hock Co.
|Field sentinel cages:
|1/2 pt cardboard can body
|1/2 pt cardboard cup lid
|Velcro cable ties 8-in x 1/2-in
|National Institutes for the Blind
|Large cotton balls (non-sterile)
|PVC 04010 0600
|Modified by cutting into 18-in length pieces and cutting half off of the end (lengthwise)
|Blue Ridge Thermalforming
|Field bioassay set-up equipment:
|60-in tread-in post
|1 ft3 cardboard boxes
|18-in x 18-in linolium tiles
|Sentinel cage transport:
|48 qt Island Breeze cooler
|16-in x 19-in. terry towels
|13 gal (kitchen size)
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