Source: Hannah Burrack, NCSU COOPERATIVE EXTENSION SERVICE, 5/29/14
 
This week growers, agents, and consultants have noticed unusually large numbers of thrips on tobacco. This is no surprise to cotton growers, who have been asking about thrips for a few weeks. Agent reports and our observations at research station and scouting locations indicated that counts between 30 and 50 thrips per leaf were common.  Accompanying these high numbers was also some classic “silver-leafing” foliar damage, which is caused by thrips’ rasping the upper leaf surface.

These observations raise the obvious question of TSWV (tomato spotted wilt virus) risk, since tobacco thrips (our most common species in tobacco) vector this virus.  The first question I asked all of the people who called me was “what do the thrips look like?” This is because of the many thrips species present in North Carolina, only about four are likely to vector TSWV, and only three of those are commonly found in tobacco.  Because thrips are so small, often the usual ones, such as the relatively large cereal thrips or distinctive striped soybean thrips are the species that people notice. However, we collected a small sample of thrips from the Lower Coastal Plain Research Station near Kinston, which all appeared to be female tobacco thrips.

Why are we seeing so many thrips in tobacco?
The thrips we are observing are are likely the third generation of tobacco thrips, and our tobacco thrips flight timing predictions were right on the mark for this year! For example, third generation thrips flights were predicted to begin in Wilson County around May 23rd, which means that we are right in the middle of third generation dispersal.  At this location, the fourth generation flight is expected to begin around June 10th, so we may observe additional thrips populations in a few weeks.  To see thrips flight predictions for your area, use our TSWV and Thrips Risk Forecasting Tool.

Winter weeds are also likely playing a role in our thrips observations. Plants such as chickweed, knotweed, and henbit are among the overwintering hosts for tobacco thrips in North Carolina, and as these plants die in the early summer, thrips are forced to migrate to new hosts.  At many of the locations I visit (including my yard), these winter annuals have accelerated their die-back in the last few weeks.

The combination of winter weed die-back and thrips generation timing likely account for our observations this week.

What do these observations mean for TSWV risk?
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 the 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.

What should we do now?
The foliar feeding damage by thrips is likely negligible.  It is occurring early in the season on leaves that will be on the lower part of the plant and subject to much more abuse before harvest.

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% incidence in an average year, with standard production practices) may want to take action in the field (see the TSWV and Thrips Forecasting Model for management recommendations).

Growers in lower TSWV areas will still likely not see a benefit of additional management efforts, especially not those directed at thrips.  Insecticide treatments for thrips are not recommended as they are short lasting and have not been demonstrated to reduce virus incidence.  This is why greenhouse treatments have become so popular for TSWV management, because while they do not necessarily kill thrips, they discourage them from feeding on plants and reduce the time they do feed.

It is also important to note that TSWV takes time to develop in the plant before symptoms are observed.  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.