Study Shows Rapid Evolution in Lab-Reared Insects
By Leslie Mertz, Ph.D.
When it comes to evolution, insects are quick-change artists. In fact, when insects are grown in laboratory conditions, some undergo noticeable adaptations in just a few generations, according to a new review of research published in the Journal of Economic Entomology.
“In some cases, we saw shifts in traits of up to 10 percent per generation, so some changes were really moving fast,” says Ary Hoffmann, Ph.D., professor in the University of Melbourne’s Bio21 Molecular Science & Biotechnology Institute. Hoffmann and graduate student Perran Ross co-authored the research, which involved an extensive search of more than 50 scientific studies to examine the speeds at which laboratory-reared insects adapt, which traits were more often affected, and whether those adaptations improved fitness or weakened it.
Overall, the researchers found that constant lab conditions spurred adaptations. “We saw changes in life history, including in how many eggs are produced and how quickly they develop. You might have a strain that produces very few eggs in the wild, but then, over just a few generations in the lab, it’s producing a considerably larger number of eggs,” Hoffmann says. At 10 percent more eggs per generation, numbers can add up fast.
This does not mean that every species experiences these changes when grown in a laboratory. Size of the lab population, conditions of the lab environment (e.g., the degree of crowding, temperature), the type of insects being reared, and how long the population has been reared—these can all affect evolutionary rates. “Some traits change quickly; some do not. There’s a lot of variability,” he says.
General findings relating to laboratory-reared insects include:
- Mosquitoes and other flies experience adaptations to lab conditions that boost their fitness (e.g., slightly larger size, faster development, more mating). Butterflies and moths, on the other hand, “can go downhill pretty rapidly,” Hoffmann says.
- Morphology doesn’t change much. Although some species may get a little bigger or smaller when raised in a controlled environment, most morphological changes were minor.
- The longer insects are reared in a lab, the lower their tolerance for temperature swings (a trait known as thermal stress resistance). “That’s important because, if a researcher is testing a population to work out whether it is susceptible to extreme temperatures and they are using laboratory stocks rather than wild insects, they could decide it’s not particularly tough when it actually is,” he explains.
- Mating behaviors can adapt quickly. While behavioral adaptations often spur reproduction in the lab, such changes can make lab-grown insects less able to find mates in the field. This can be especially problematic for sterile-release programs.
Understanding the extent of adaptions is important for several reasons, according to Hoffmann. “We rear quite a lot of insects these days for the purpose of biocontrol,” he says, explaining that these include parasitoid wasps that are released to attack certain farm pests. Such biocontrol efforts help reduce reliance on pesticides. “There’s a large number of these lab-reared insects going out in the field, particularly in situations like glass houses [greenhouses], so you want to make sure that they have very high fitness and are going to perform very well. If, however, you are using insects that have started to become adapted to laboratory conditions, then you may find that that fitness and performance decrease, and that can be a real issue.”
Likewise, control programs also raise large numbers of sterile male insects, especially mosquitoes, as a way to control the breeding of potentially disease-carrying females. “Millions and millions of sterile males are being released over thousands and thousands of hectares, and those sterile males have to compete with males out in the field. If the lab-reared, sterile males start losing that competitive ability, then you have to start releasing a hell of a lot more of them to have an impact, so it’s a pretty critical issue,” Hoffman says.
Those rearing insects would do well to keep in mind the potential for lab adaptations, and they might consider either replenishing their stock at appropriate intervals or cross-breeding the lab population with wild individuals to ensure they have the healthiest and most capable insect stock for release programs and for research studies, Hoffmann advises. “This is extremely important, and just something you really have to do.”
In addition to this review study, Hoffmann’s research group is preparing to publish another paper on the rate of adaptation of the mosquito species Aedes aegypti, which spreads yellow fever, Zika, and a variety of other pathogens. “We were expecting a fairly high rate, but it’s at the lower end,” he says. He postulates that the low rates may stem from the fact that these mosquitoes normally live near human habitation, so they are already accustomed to a somewhat artificial environment. “It’s quite an intriguing finding,” he adds.
Hoffman and his group are also interested in a couple of parasitoid wasps to be released as a tactic to combat certain leaf-mining insects, and he affirmed that they will indeed be taking adaptation rates into account. He remarks, “Our newly published review paper reinforces the fact that we, like everyone else, have to take lab adaptation seriously and consider it very carefully. So, yes, adaptation rates are very much on our agenda.”
Journal of Economic Entomology
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.