Tobacco growers and consultants reported unusually high thrips numbers in North Carolina tobacco the last week of May, according to the state’s tobacco entomologist. Should growers be more concerned about tomato spotted wilt virus infection in the state?
In her May 29 blog post, Hannah Burrack, NC State University Extension entomologist, says thrips counts at the time ranged from 30 to 50 thrips per tobacco leaf, along with the classic signs of silver-leafing foliar damage.
The higher thrips numbers likely due to third generation tobacco thrips, which are flying on mark with earlier predictions. Flights were predicted to begin in Wilson County around May 23. In Wilson County, the fourth generation flight is expected to start June 10, so additional thrips populations can be expected in the next few weeks. This raises concern for TSWV risk. Tobacco thrips vector TSWV.
“TSWV infection in a plant is a complicated process. First, a larval thrips needs to feed on an infected plant, then the adult thrips must feed on susceptible a host plant to transmit the virus. In tobacco, this is further complicated by the fact that plants become more resistant to infection as they age. A six week old plant, for example, is generally less likely to develop a systemic (and therefore, damaging) infection of TSWV than a three week old plant.
The first link in the chain requires that there be TSWV infection in winter weed hosts so that larvae can acquire the virus. Last year was a relatively low TSWV incidence year, suggesting that virus presence in winter weeds may also be low. This would mean that fewer of the thrips moving into our fields right now are capable of transmitting TSWV.
The second link requires a susceptible host plant. We had a prolonged transplant period this spring and early summer, which means there is a lot of variability in plant age. Younger plants (less than six weeks old) are at higher risk of infection than older plants.
Predictions made by our TSWV and Thrips Forecasting Model still suggest that this will be an “average” year for most locations. This model takes into account last year’s thrips populations and this year’s weather conditions to make its prediction. This average observation may be due to the fact that we had relatively low thrips numbers and relative little virus incidence last year.
Growers in high-TSWV risk areas (greater than 10 percent incidence in an average year, with standard production practices) may want to take action in the field.”
She adds growers in low-TSWV areas will likely not see benefit from more management efforts against TSWV, especially thrips related. Insecticide treatments are too short lived to do much good.
TSWV takes time to develop in the plant, she says, before symptoms appear. This means that any diseased plants observed in fields right now are likely due to infections from a few weeks ago and that it take a few weeks from now for any infections due to this thrips flight to develop.