Tuesday, August 20, 2013

Early August Insect Update

It has been a number of days since we last reported on Alabama insects. So what has been happening? First of all, August has been dominated early-on with little insect activity other than the bug complex in cotton. Second has been the weather, we had sun yesterday, August 19, for the first time in about 10 days. During this period, rainfall has been measured in inches varying from low single figures to double digits, depending on the location. Up to 4 inches in a 24 hour period has been common. This has resulted in bridges and roadways being washed out and public schools closing during the beginning of the fall term. Agriculturally wise, sprayers have been unable to get into fields and aerial applicators are over booked. Fortunately, insects have overall been low with just a few exceptions. The bug complex has been over threshold in many but not all cotton fields. In the past few days the caterpillar complex has built to damaging levels in soybeans. Some fields have been sprayed by plane for the lep complex, primarily loopers and velvetbean caterpillars. The heaviest infested fields are those where growers just preventatively added a pyrethroid when they were applying a fungicide.

It has been my experience in research plots that an application of a pyrethroid just prior to an infestation of foliage feeding caterpillars is like throwing gasoline on a fire. We usually see much higher numbers of soybean loopers where a pyrethroid application has been made within the previous 10-14 days. My advice to growers is that if they feel they must add something to their fungicide, just add 2 or 3 ounces of a product like Dimilin which does not disrupt the beneficial insects.

Once we have a damaging level of foliage feeders the first thing we need to do is to quantify the species present and the numbers. Five to seven caterpillars per foot of row will usually result in 30% or more foliage loss and therefore require controls. Treatment thresholds for sweep nets on drill beans are not quite as well defined. However, 2 to 5 per sweep would likely require controls. As to species, velvetbean, green cloverworms, corn earworms (podworms) and stink bugs can all be controlled with a pyrethroid. However, if many soybean loopers are in the mix, then one of the newer lep materials will be required. These are, in chemical alphabetical order: Belt, at 2-3 oz.; Steward at 7 oz.; Intrepid at 4-6 oz.; Prevathon at 18-20 oz.; tracer or Blackhawk at 1.5-2 oz. per acre.

One additional thing should be added about soybean looper control, especially if application is by air. Looper eggs are deposited on the lower leaves of the plant, therefore they infest the plant deep within the canopy. As the larvae mature they feed upward through the canopy. In order for any of the newer chemistry to be effective, the leaf containing the droplet has to be eaten. Therefore, it may take several days to fully suppress a looper population. It has been my experience that the surviving  larvae behind application will be early instar larvae found lower in the canopy. Most of these looper insecticides are very rainfast and have long residual on the leaves, so don’t panic if you see a few small loopers behind an application.

I will be on research farms for the next several days and will get back soon if we see or hear of damaging levels of insects developing.