[Technically Speaking]

Ever wonder why one product works better than another? This is not an uncommon theme when pest management professionals get together — asking each other what product seems to work best for a given pest.
Manufacturers list a plethora of pests on their products’ labels and their representatives comment on the efficacy of their product. My initial response to these efficacy claims, particularly if they sound too good to be true, is, “Show me the research data to back it up.” If the data is testimonial in nature — forget it. If the data was generated in the lab and/or field by independent research I feel more comfortable with the results; however, the gold standard is how the product works in the field in professionals’ hands.
There are many factors that determine how a product that mitigates pests will perform in the field. This article will address some of the key factors affecting product performance.

RESISTANCE. The most well documented factor affecting product performance in our industry is resistance. Resistance in insects typically takes two forms, physiological and behavioral. Physiological resistance is most often associated with pesticides, although it could involve heat, cold and other abiotic factors. Behavioral resistance results when there is a change in a previously “normal” activity, such as German roaches feeding on a bait.
The good news is that resistance only affects a limited group of pests, e.g., German cockroaches, house flies, mosquitoes, stored product pests and most likely bed bugs. These groups have a high reproductive potential (large populations can occur in a relatively short period of time), they have a short developmental time (egg to egg), a lot of reproductive females and males in the population, and a lot of eggs per female. Thus survival of pesticide exposure by more fit individuals can lead to genetic changes in the population, which increases the population’s tolerance of pesticide exposure.
In contrast to this are two major pest groups, ants and termites, that are unaffected by resistance. While the reproductive potential in these two groups is high, the gene pool is small because most eggs are the product of just a few females and males. Genetic change is slow and the ability to develop resistance to pesticides is virtually non-existent.
Another resistance phenomenon is cross-resistance — resistance to a product to which the population has never been exposed. The most recent and well documented case of cross resistance occurred in German cockroaches when Ficam (bendiocarb) was introduced in the 1970s. The detoxification mechanisms that evolved in German cockroaches as the result of exposure to chlorinated hydrocarbons were equally effective in detoxifying bendiocarb. Resistance doesn’t disappear overnight, so the absence of bed bugs for the past 35 years doesn’t mean that resistance developed years ago is gone.

REPELLENCY. After the loss of chlordane, and with the exception of Dursban, the termite industry relied on repellent pesticides (pyrethroids) for their termite control. The success of these products, e.g., esfenvalerate, permethrin, cypermethrin and bifenthrin, in repelling termites is a testimony to their effectiveness as repellents. The logical extension of this principal would be that most other species of insects are similarly repelled, however this is not the case or these products would have failed as residual insecticides for cockroaches, ants and other structural pests.
It is likely that there is some repellency associated with these products but the degree to which they may be repellent is affected by formulation, final concentration, and active ingredient. The target pest’s response to a given product may be influenced by the route of exposure, e.g. contact, inhalation or ingestion. The insect’s receptors and where they are located may be another factor.

TARGET PEST. Morphological structure plays an important role in the efficacy of a pesticide because it affects the target pest’s interaction with the toxicant. For instance, arthropods with piercing/sucking mouthparts, such as bed bugs, mosquitoes and ticks, virtually rule out the use of baits for their control because they have no means of consumption. Insects with chewing mouthparts, such as cockroaches, ants and termites, and with sponging mouthparts, such as filth flies, can be effectively controlled with baits.
If there is no physiological resistance, virtually all arthropods are susceptible to contact insecticides. However, the amount of material that must be applied and its concentration may vary greatly mainly due to the degree of absorption through the cuticle. Some insects, such as cockroaches, have a thick cuticle with a heavy waxy layer that inhibits the penetration of pesticides. On the other hand, the cuticle of insects, such as termites, is not melanized (hardened), rendering them more susceptible to contact insecticides at much lower concentrations. The fact that bed bugs can survive long periods without feeding and not desiccate (dry out) may be due to a thicker cuticle and/or waxy layer. Control failures may result because the concentrations of available products are not sufficient to penetrate the cuticle or there is not enough product present to effect control.
Spiders present an interesting challenge. They are not affected by baits. They virtually walk on their toes (tarsal claws) and do not contact residual pesticide applications. Even if they did they do not groom and therefore cannot physically ingest these products. Thus control is best affected by direct contact sprays or aerosols that can be inhaled.

FORMULATION. Considering these factors, and these are but a few, the choice of formulation for the target pest is critical. The choice of formulation is also driven by many abiotic factors that affect product performance, such as moisture, heat, cold, cleaning agents and surface composition. For instance:
• Some pyrethroids have negative temperature coefficients, i.e., they are more effective at lower temperatures.
• Typically, microencapsulated insecticides and dusts are going to provide a better residual on porous surfaces.
• Most dusts and ECs do not hold up in moist situations and are easily removed by washing.
• Aerosols provide no residual activity.
• Baits are very specific.
Product effectiveness is affected by many factors that must be considered when selecting a product for use in every situation.

The author is president of Innovative Pest Management, Brookeville, Md. He can be reached via e-mail at rkramer@giemedia.com.