An onion thrips larva (Thrips tabaci) on an onion leaf. In a recent study, researchers at Cornell University found that onion thrips infected with Iris yellow spot virus live longer than thrips that are not infected. Both onion thrips and Iris yellow spot virus cause damage to onion crops. (Photo courtesy of Ashley Leach)

By John P. Roche, Ph.D.

Onion crops are vulnerable to a virus known as Iris yellow spot virus (IYSV), which they acquire from insect pests called onion thrips (Thrips tabaci). IYSV causes necrosis in onion plant tissues and leads to millions of dollars of crop losses each year. Thrips can pick up the virus from infected onion plants when they are larvae, and, once infected, the thrips can spread the virus to other onion plants throughout their lives. Previous research discovered that infection with plant viruses in the genus Tospovirus increased lifespan and fecundity in a different species of thrips called flower thrips. Recently, researchers at Cornell University conducted an experiment to test if IYSV affects lifespan and fecundity in onion thrips.

The research team—led by Cornell graduate student Ashley Leach, along with colleagues Marc Fuchs, Ph.D., Riley Harding, and advisor Brian Nault, Ph.D.—compared the lifespan and fecundity of onion thrips that were infected with the virus with thrips that were not infected. To do so, they gathered onion plants from fields in which IYSV was present, maintained the plants in the lab, and exposed thrips to the onion plants so that they could pick up the virus and lay eggs. They then allowed the resulting eggs to develop to pupation. They removed pupae and placed them on discs of cabbage leaf as a growing medium. When onion thrips emerged from pupation, the researchers monitored their survival and reproduction every 24 hours throughout their lifespan. All thrips in the experiment were female, and their eggs developed into larval thrips parthenogenically (without fertilization).

One hundred and forty-nine thrips were used in the experiment. The investigators determined whether or not individual thrips had become infected with IYSV by testing for presence of a IYSV nucleoprotein gene using a technique known as reverse-transcriptase polymerase chain reaction. Previous studies did not determine whether thrips were infected with tospoviruses; by determining infection status, Leach and colleagues created greater statistical power than was available in previous work.

Of the 149 thrips tested, 77 percent were infected with IYSV. The investigators found that thrips infected with IYSV lived 3.6 days (or 22 percent) longer on average than uninfected thrips. They found no significant difference in fecundity, however, between infected and uninfected thrips. The results of the study were published in late May in the open-access Journal of Insect Science.

Onion thrips (Thrips tabaci) are a double threat to onion crops because they can cause direct damage via feeding and transmit Iris yellow spot virus to plants. (Photo credit: Bruce Watt, University of Maine, Bugwood.org)

Onion thrips (Thrips tabaci), shown here on a flower of a pepper plant, measure just 1-2 millimeters in adult form. (Photo credit: Metin Gulesci, Bugwood.org)

Cornell University entomology graduate student Ashley Leach researches integrated pest management strategies for onion thrips and Iris yellow spot virus in the lab of professor Brian Nault, Ph.D. (Photo courtesy of Ashley Leach)

“Understanding the ecology of infection is important to developing management plans for plant viruses in the field,” Leach says. “IYSV is an important virus in onion production, and understanding how onion thrips are impacted by IYSV can help us better understand the greater epidemiology of IYSV.”

The increased survivorship is doubly significant from an agricultural standpoint. Onion thrips feed by making a hole in onion leaves and sucking out plant juices. This reduces the ability of the plant to photosynthesize, causes loss of water, and makes an entry point for plant diseases—all of which are detrimental to onion plants and can cause crop losses. By increasing the survivorship of onion thrips, onion thrips can do more damage to onion plants. In addition, onion thrips feed on multiple plants, and sometimes multiple crop fields, so, by increasing their life span, they have a better chance of infecting more onion plants with IYSV, which is also detrimental to the onion plants.

One intriguing aspect of Leach’s findings is that only 77 percent of the onion thrips that were given access to infected onion plants became infected. Why did some thrips become infected and some not become infected? Is it due to differences in physiology, feeding behavior, or other factors? “Past studies have shown that IYSV tends to be unevenly distributed throughout the plant, so the feeding location of thrips may significantly alter the likelihood of that thrips acquiring the virus,” Leach says. “It’s possible that we can have two thrips feeding on the same plant, and one may acquire the virus and the other may not, according to where each was feeding during larval development.”

Another intriguing question that arises from the study: How does IYSV increase longevity in onion thrips? “Previous literature has provided some great hypotheses for why IYSV might impart increased longevity, including changes in the thrips midgut function from the tospovirus infection,” Leach says. “But the effect of tospoviruses on thrips tends to yield inconsistent results, and the effects are likely to differ based on the specific species and populations studied.”

Previous studies have observed that tospoviruses can cause changes in variables such as feeding behavior, development timing, and diet choices in thrips. In their paper, Leach and colleagues mention that, to further our understanding of how tospoviruses can cause such changes, comparative research is needed on different isolates of IYSV and different populations of thrips.

“Multiple studies have been published looking at the effects of tospoviruses on thrips, and I think these studies could serve as a template for additional research with IYSV and onion thrips,” says Leach. “For example, it would be useful to examine if there is a difference in feeding behavior between infected and non-infected onion thrips and how those changes may impact the greater ecology of onion thrips in onion.”

The subject of tospoviruses and thrips is fascinating in that both species damage crops and the two species interact with each other in a way that increases the damage each species can cause to crops. Future research promises to add to our understanding of these complex virus-insect-crop interactions and provide ongoing improvements in management of both tospoviruses and their insect vectors.

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 195 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.