Moderate greenbug populations have been found in most area wheat fields and are beginning to colonize the plants as evidenced by mature greenbugs surrounded by the smaller immatures (nymphs). The local greenbug populations have not been established very long, but it would be prudent to keep a close eye on them.
When conditions are ideal for reproduction, populations can increase up to 20 fold in one week, although a five or six-fold jump is more likely. Greenbugs cause more damage than other aphids in wheat because they inject a toxin into the plant when they feed.
Heavily damaged plants may not recover, and do not produce a normal grain yield. Heavy feeding causes localized leaf tissue damage, which can ultimately cause death of the leaf. If several leaves are affected on small plants, the whole plant may die.
Growers should consider the following factors before making a greenbug spray decision:
- Plant size – Large plants can tolerate up to three times as many greenbugs as small plants that have not completed the tillering process.
- Fertility – Plants that are lush and actively growing can tolerate higher populations than plants that are under stress from inadequate fertility or unfavorable growing conditions.
- Time of year – Late fall and early winter greenbug populations can explode, increasing by as much as twenty-fold in one week. This is because greenbugs thrive and reproduce rapidly in cooler temperatures that do not favor beneficial insects. Our most important beneficial, a tiny parasitic wasp, becomes active as temperatures increase. In late February and early March, this little wasp begins to parasitize the aphids, which rapidly reduces greenbug populations.
Adding an insecticide to the topdress nitrogen application is a common practice in this region but is not sound. Topdressing before the middle of February can result in excessive nitrogen loss through leaching and denitrification, which would reduce potential grain yield by 6 to 15 bushels per acre.
When greenbugs are a problem this early, a better option is to spray the greenbugs and hold off the nitrogen application for a few weeks. Optimum time to dribble the nitrogen fertilizer solution on the crop would be between Feb. 15 and March 1. With wheat at $3.00, why give up $18 to $45 in wheat sales per acre just to save an application charge?
Jim Swart is Extension IPM specialist, and Don Reid is an agronomist with Texas A&M University-Commerce.