The Evolution of Insect
Resistance to Insecticides and
Other Deterrents
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.
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.
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.
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.
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.
Methods
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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).
Results
Affected Evolution of
Resistance:
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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:
•
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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…continued
Dominance plays a role in the
evolution of resistance,
especially at lower
concentrations of toxin.
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
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.
Societal & Earthly Issues
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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.
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