Soybean thrips infected with the soybean vein necrosis orthotospovirus reproduce better than uninfected insects
A virus is a threat to many crops but, in a complicated relationship, the soybean vein necrosis orthotospovirus is an unexpected benefit to soybean thrips.
Researchers at Penn State’s College of Agricultural Sciences have discovered that, when soybean thrips are infected with SVNV, they thrive longer and reproduce better than thrips that are not infected.
SVNV is a critical viral disease to soybeans and can be seed-borne or insect-vectored. The disease affects seed quality and oil content and, according to the researchers’ report, increased damage can be seen in late planted soybeans. The disease is widespread, and almost all soybean-growing states in the United States are affected.
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When a plant is infected with SVNV, the pathogen attacks the leaf veins, turning them yellow. The infection spreads to other leaf parts where brown lesions develop. Oils and proteins in the seeds are reduced, affecting overall seed quality and weight.
The virus is spread when soybean thrip larvae feed on an infected plant’s leaves. Then, as adults, they spread the virus to other plants through their saliva.
The disease was discovered in Arkansas and Tennessee in 2008 and has now spread to 12 states, as well as Ontario.
“A life table study was conducted to determine the effect of soybean vein necrosis virus on thrips,” said Asifa Hameed, who led the study while completing a doctoral degree in entomology at Penn State. She is now an assistant entomologist at Mango Research Institute in Multan, Pakistan.
In the study, parameters such as life history, survival, mortality, population doubling time, fertility and fecundity were assessed and comparisons made between SVNV infected and uninfected populations.
The researchers collected soybean thrips from fields and released them onto soybean plants in the lab. The thrips and plants were monitored regularly for SVNV infection using PCR testing.
The thrips were monitored through two generations and observations were recorded of their lifespan, mortality, fertility and reproduction. Researchers found that lifespan in thrips not infected with the virus was shorter than that of the infected thrips, which survived better, lived longer and had an increased reproduction rate.
“We found that infected thrips tended to produce more offspring,” said Hameed. “On average, uninfected females produced … 84 eggs, while those infected with SVNV produced 89.”
The relationship between the infection of plants, the pest and the virus is most likely an evolutionary one, said Hameed. Given that virus-infected plants increase the activity and survival of thrips, the population of those infected insects increases. With the decrease in health of virus-infected plants, pests move to healthy plants where their feeding habits expose plant leaves to their infected saliva, effectively spreading the disease to the virus’s advantage.
“I found during my PhD that virus-infected plants, while poor quality, actually released higher amounts of volatiles, which resulted in the attraction of uninfected thrips,” said Hameed.
The SVNV virus is able to replicate itself in thrips and, through their feeding habits, transfer into the crop plants. Hameed said that, due to the virus’s ability to bind with receptor insect gut proteins, the virus can replicate and propagate in the plants as well. Other crops vulnerable to SVNV include field pumpkins, cowpeas, mung beans, tobacco, and pulses.
An invasive plant called kudzu found in the southern U.S. states and native to Asia has been found to be an overwintering host for both the thrips and the virus.
SVNV has been found in the United States, Canada and Egypt, as well as Pakistan.
“A survey was done with Pakistani farmers and scientists to determine whether they knew about this devastating disease,” she said. “Unfortunately, most of them were unaware of it.”
There is no known treatment for SVNV in plants. However, thrips can be managed with a few options. In addition to pesticides, biological control may include the use of predatory mites or orius bugs, also known as minute pirate bugs, that are aggressive thrips predators. Geocoris insects are also valuable predators.
Hameed said future research is needed to determine the overall effect of the virus on the genomics of infected thrips compared to uninfected insects.
The study was published in the journal Insects.
