With cotton planting seriously getting under way during the first couple of days in May, thrips have not yet played their hand.
This will probably change during the third week in May for many North Carolina cotton producers when the residual effectiveness of the seed treatments begins to play out.
So far, our temperature and moisture conditions over much of the state, though a little cold recently, have been favorable for both seedling emergence and insecticide uptake. This was certainly the case at Upper Coastal Plain Research Station near Rocky Mount where good stands emerged within 6-7 days at all six our early May 1-2 planted thrips tests.
Thrips levels following one of the seed treatments there averaged only 1/10 of one adult and no immatures, while the untreated check in the same test averaged 1.2 adults and 1/10 of an immature thrips in one of our tests on Wednesday May 9. With these low thrips levels, we could see no difference between any of the treatments at 8 days after planting.
Hopefully, that’s the case over most of the state for cotton planted last week. Remember, however, that next week’s report could be much different as thrips levels build and the residual of seed treatments begins to run out.
Easy to talk about on paper, thrips counting and seedling damage assessments can be less than straightforward in the field.
Following emergence, seedlings can appear “beat up” for a number of reasons, in addition to thrips damage.
For cotton that is scouted following a seed treatment, granular insecticide or and in-furrow spray, always be sure to focus attention to the developing bud area and confirm the presence of live thrips with either a hand lens or by beating several seedlings onto a flat surface to look primarily for the small yellowish immatures.
The presence of immatures often indicates that the seed treatment or other insecticide has begun to run out.
The very tiny 1stand 2ndinstar immature thrips can be difficult to see with the naked eye due to their small size and their presence in the hidden folds of bud tissue.
Figure on a treatment threshold of approximately an average of 1 immature thrips per true leaf per plant (for example, 3 true leaf seedlings could tolerate up to 3 thrips per seedling).
Alternatively, one can use an average of 2 immature thrips per plant as justification for a foliar spray.
If one is scouting for a possible second application, for example in 2 to 4 leaf cotton, be sure to disregard leaves that may have been previously damaged. They’re not going to get better. Concentrate again for the presence of live thrips in the bud area and the newest unfolding leaf. If a foliar spray is needed, avoid pyrethroids.
One clue that seedlings are growing out of thrips damage is the presence of a smooth, shiny newest leaf in the bud area. With very few exceptions, cotton seedlings with 5 true leaves cannot be damaged by thrips, even if thrips are present.
In the photo, as one can see from the nicely expanded cotyledons, the seedlings got off to a good start following a seed treatment. However, with the residual for the seed treatment only lasting approximately 2.5 weeks, the next four true leaves are heavily damaged by thrips.
With the fifth true leaf just coming out, the seedling may be just getting to the point where a foliar spray is not needed because cotton at this stage can tolerate moderate thrips levels without further economic damage. A “revenge spray” at this point will not salvage the previously damaged leaves or plants.
Other seedling pests
So far we have had no reports of false chinch bugs, burrower bugs, vegetable weevils, sugarcane beetles, slugs or grasshoppers damaging cotton in North Carolina.
Because we are still on the front end of most possible insect problems, however, be sure to report signs of damage from the above or other pests to [email protected] so that we can pass this on to others in next week’s insect update.
Weekly cotton insect update tapes are also available each Wednesday at our Cotton Insect Corner web site.
(You might also be interested in Growers in 'thrips central' ready for 2012 battle).