Flexible Reproduction ‘Mite’ Explain Invasion Success
By Melissa Mayer
Invasive species are an expensive problem, with invasive insects alone racking up at least $70 billion a year in costs around the globe—and it’s only going to get more expensive. Scientists expect an 18 percent increase in arthropod invasion by 2050, according to climate change projections.
In a new paper published in January in the Journal of Economic Entomology, researchers from Massey University’s School of Agriculture and Environment in New Zealand looked at the reproductive strategies used by Tetranychus ludeni mites, sometimes known as red spider mites, to invade every continent except Antarctica. It turns out mites successfully inbreed and tune their reproduction to their circumstances, allowing them to respond flexibly to new environments.
Tetranychus ludeni mites are native to Europe and now serve as important invasive pests of plants all over the world. Like all arthropod invaders, mites face barriers during the introduction, establishment, and spread stages of biological invasion—and overcoming those barriers can make or break an invasion.
“The species that have become invasive pests must have specific features, particularly strategies for survival and reproduction to facilitate their successful invasion,” says Qiao Wang, Ph.D., senior author on the study and doctoral supervisor for Peng Zhou, the lead author who performed the research. “So far, quite a few spider mite species have become invasive pests worldwide, but underlying mechanisms are not that clear.”
At the introduction stage, invaders may struggle to find suitable mates, but T. ludeni mites have a reproductive trick up their many sleeves. The mites are haplodiploid, so a single female invader can lay unfertilized eggs that hatch into male mites—and then she can mate with those sons to produce fertilized eggs that hatch into female mites, which can go on to mate with their brothers.
The downside of this strategy is that inbreeding can produce less fit offspring and, eventually, population collapse for the invader. Some evidence already suggested that this isn’t a problem for spider mites, but the researchers tested this out to 11 generations to see if the line held up.
A Delicate Touch
The team reared two colonies of T. ludeni mites in separate rooms. Then, Zhou individually mated the mites in three ways over 11 generations: mother-son mating, brother-sister mating, and outbreeding (i.e., mites from different colonies).
Adult spider mites are less than 1 millimeter in size, so this was a careful task. “Mites of different sexes and sources were individually transferred with a fine brush under a dissecting microscope to allow mating to occur according to treatments,” says Wang. “Mites are very fragile so the transfers must be very delicate.”
For each female mite, the team recorded the number of eggs produced before and after mating as well as the total number and percentage of daughters.
The researchers were also interested in how mating influenced reproductive fitness without inbreeding in the equation, so they performed a separate experiment. In this one, they counted how many eggs and daughters were produced over a lifetime for female mites who never mated, female mites who mated right away, and female mites who were kept from mating for 10 days.
Haplodiploidy for the Win
It turns out inbreeding is a solid strategy for T. ludeni mites. The team saw no evidence of reduced fitness—called inbreeding depression—over 11 generations. It’s possible that small groups of haplodiploid animals are protected from undesirable recessive alleles.
It also looks like mites can quickly adjust their reproductive resources to account for uncertain times. Before mating, young female mites laid lots of eggs to produce males for mating. If they were never allowed to mate, they scaled back egg production to prolong their lives in the hope of future mating.
Overall, female mites that never mated laid the least number of eggs over a lifetime—but there was a twist. Those mites produced bigger sons, a strategy that could pay off down the road if those sons produced more daughters than their smaller peers.
All female mites cranked up egg production immediately after mating. Those with delayed mating produced fewer eggs overall but a higher proportion of daughters compared with those allowed to mate right away.
This new data may help explain why T. ludeni mites have successfully invaded so many different habitats—and may help entomologists better predict and manage biological invasions by other haplodiploid arthropods.
“Although potentially invasive species may have different strategies to invade new environments, this study indicates that understanding a species’ reproductive strategies may be a good start for invasion risk analysis and predictions,” says Wang.
Journal of Economic Entomology
Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: firstname.lastname@example.org.