Raj Saran, Ph.D., global product biology expert at the Syngenta Vero Beach Research Center in Florida, explains how he and fellow researchers test pest control formulations on particularly stubborn pests.  In your research, which insects tend to be the hardiest and most resistant? Resistant strains of German cockroaches, bed bugs, house flies and mosquitoes are difficult. Not only are they tolerant to practically applied label rates, but they also have unique behaviors that contribute to the challenge. For instance, German cockroaches may be tolerant and bait-averse at the same time, and it takes some creativity to separate these two mechanisms. Bait-aversion is more of a behavioral issue where tolerance to pesticides is physiological or genetic. Bed bugs are also tough. Direct application to bed bugs in a petri dish may work and resistant ones may also die at reasonable rates or in combination with different modes of action and synergists. However, when you apply the same insecticides on different surfaces such as carpets, mattress covers, wood, etc. you can observe very different results. This may be because bed bugs don’t pick up lethal doses from these surfaces easily, or worse, resistant bugs have refractive (thick) integuments so they can’t easily absorb insecticides through their thickened cuticles. In this case, the mechanism of resistance is more difficult to figure out. Are there environmental factors you must account for with certain pests? What about hard-to-reach harborages? Yes, the two most important factors in test arenas are temperature and humidity. We record temperatures and relative humidity (RH) values every 30 minutes to monitor major fluctuations. As we know, most of these insects do well in hot and humid situations, so we try to keep RH between 50-60% ±5% and temperatures at 77 F ±1. We must be very careful with temperature because some insecticides may show a negative or positive correlation with a slight change in the temperature at which we run our assays. Harborages are generally not an issue when we run simple efficacy tests. We provide a rather simple harborage of cardboard. However, when we are testing the potency of insecticides on full colonies, such as a small ant colony with queens and brood, then we must provide a more complex and bigger harborage to mimic the natural conditions. Sometimes, we need to acclimatize insects before running actual tests. Harborages become a challenge only when we cannot easily observe the status of the brood and queen inside. What kind of adjustments do you make when an experiment shows an insect hasn’t been affected? Is it as simple as just increasing the active ingredient (A.I.)? That’s a good question--it is a very interesting challenge. Increasing the quantity of A.I. is an obvious reaction, and it depends on what type of assay we are running. For example, are we looking at intrinsic toxicity using technical grade A.I. or directly spraying diluted product on the insects or testing formulation efficacy on different surfaces? In any case, we seldom use a single rate in our trials. We generally have two to three rates for compounds we have worked with before. But with newer compounds, we use a range of rates (five to eight). We don’t look at just increasing the A.I. rate but also the delivery process. What formulation and solvent are we using? Is the solvent dissolving the A.I. completely or is the solvent capable of penetrating the cuticle? In the case of formulated products, the particle size is important depending on the surface it is applied to, so in such cases we use different iterations of the same formulation or use different formulations (soluble concentrate, microcap, wettable powder, water soluble granules etc.) containing the same A.I.
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