The Evolution of Insect Resistance to Insecticides and Other Deterrents

1. The Evolution of Insect Resistance to Insecticides and Other Deterrents By Phil Hartger

2. General Overview of Resistance to Insecticides/Pesticides • In the 1940s, resistance initially began to occur because farmers started to use synthetic pesticides. • In following the “rules” of evolution, insects and other pests have grown over generations to do the only thing important to them……survive. • As farmers become more desperate to kill pests that are killing approximately 13% of all crops, the farmers use more and more pesticides. • This procedure actually hastens insect evolution because resistant individuals come to dominate and propagate exponentially.

3. What’s the problem with widespread insecticide use? • In addition to the obvious problem of insecticide resistance, widespread use of insecticides are often hard on the environment, spreading toxic effects on ecosystems and human health alike. • By using insecticides, we as humans are creating selection pressures that affect not only insects but us as well. • As more and more insects develop resistance, use of insecticide also becomes more and more expensive. • Because these insecticides are so harsh, agricultural researchers have developed less harsh methods, such as Insect Growth Regulators (IGRs), to combat infestations.

4. How does resistance come about? • Mutations in insect populations lead to a resistance allele (R). This allele is usually dominant over other alleles coding for susceptibility. • Therefore, haploid individuals in haplodiploid populations such as the whitefly usually only need one copy of the allele to confer resistance. • The allele does not usually become fixed in populations that aren’t exposed to insecticides because it insects that contain the resistance allele are often less fit in environments that are not treated with insecticides.

5. What can researchers do to help deal with insecticide resistance? • While stopping insecticide resistance completely is beyond any capabilities researchers have at the moment, there is much work being done to produce optimal use of insecticides and other deterrents. • Because insecticides are often detrimental to the environment, scientists look to develop less environmentally harmful methods as well as sustain insect susceptibility to insecticides to reduce extraneous use of ecologically harmful deterrents. • “Natural” insecticides present in transgenic plants are currently under hot research as well.

6. The Paper • In “Modeling Evolution of Resistance to Pyriproxyfen by the Sweetpotato Whitefly (Homoptera: Aleyrodidae), Crowder et al. create population models of the Whitefly to interrogate the evolution of its resistance to the insect growth regulator (IGR) pyriproxyfen. • This particular study examined whitefly resistance to the IGR on cotton plants. This particular whitefly is one of the most prominent pests worldwide, attacking numerous kinds of crops and evolving resistance to deterrents quickly. • The researchers, through evolutionary modeling, want not only to look at various aspects of the evolution of resistance, but also to optimize when they apply IGRs.

7. Methods • In simulating the population dynamics to view how the whitefly will potentially react to Pyriproxyfen, Crowder et al. mathematically factored in such things as crop phenology, whitefly longevity, movement, proportion of males in the population, and mating. • Resistance was coded for by one gene and two alleles, S and R. • Simulations were run for 100 years, the time to resistance being the state where R grew to a frequency of .5. • Used mathematical “stuff” as well as natural observations to figure aspects of population dynamics such as eggs per day (below).

8. Affected Evolution of Resistance: Increases in toxin concentration spread the resistance allele (R) faster. As initial allele frequency increased, so did evolution of resistance. Increases in the dominance of R decreased the evolution of resistance to pyriproxyfen. Increased susceptibility of both R and S males compared with RR and SS females increased the rate of evolution of resistance. As fitness costs increased, the evolution of resistance decreased. No effect on the rate of evolution of resistance: Movement of the Whitefly to “refuges” and new growth had little effect on the evolution of resistance. With medium and high concentrations of IGR, the proportion of haplodiploid males had no effect on the evolution of resistance. Results

9. Results…continued As you can see, as the proportion of males in the population increased at low levels of IGR, the time to resistance increased until reaching a proportion of .5, then decreased Dominance plays a role in the evolution of resistance, especially at lower concentrations of toxin.

10. Conclusions • By using evolutionary modeling, Crowder et al found that resistance can evolve in as little as 20 years. • Many of the effects such as proportion of males in a haplodiploid population and dominance of females played a role in the evolution of resistance because of population dynamics. • “Refuges,” or areas that are not treated with pesticides, may be important in sustaining susceptibility to pesticides and IGRs, therefore decreasing the evolution of resistance • To delay resistance and maintain susceptibility, agricultural producers may benefit from treating fields during periods of fast growth, when refuges are increasingly present.

11. Societal & Earthly Issues • In addition to destruction of cotton and economic loss of producers, clothing made of such material may be more expensive to markets. • Many of these “pests” infest food crops as well, creating economic problems and raising prices not only for the producers, but consumers of food as well. • Similar to the thought that we want to keep our homes clean, we want our larger home, the Earth, clean as well. • IGRs are an important step in conserving our environment, but we still want to reduce use of deterrents to prevent resistance and reduce any potential effects on wildlife and ecosystems.

12. References • Crowder, DW, Author, Reprint Author Crowder David W. Crowder, David W. , Carriere, Y, et al. Modeling evolution of resistance to pyriproxyfen by the sweetpotato whitefly (Homoptera : Aleyrodidae)J ECON ENTOMOL 99 (4): 1396-1406 AUG 2006. • Denholm et al. Challenges in Managing the Insecticide Resistance in Agricultural Pests, Exemplified by the Whitefly Bemisia Tabaci. Philosophical Transactions: Biological Sciences 353 (1376) pp. 1757-1767 Oct., 1998. • Futuyma, Douglas. Evolution. Sinauer Associates January 2005. • Aldbridge, Susan. “Insecticide resistance: from mechanisms to management.” Association of British Science Writers Ocober 1998. http://www.absw.org.uk/Briefings/insecticide_resistance.htm