SOYBEAN INSECT UPDATE : GRASSHOPPERS AND THREE-CORNERED ALFALFA
HOPPERS IN SOYBEANS, TOBACCO BUDWORMS IN MISSISSIPPI SOYBEANS
Tim Reed, Extension Entomologist
August 5, 2010
During the first week of August I inspected soybean fields in Tuscaloosa county that had significant numbers of both grasshoppers and three-cornered alfalfa hoppers (3CAH). Grasshoppers feed primarily on the foliage but they can also feed on pods. No noticeable pod feeding was found on the plants the farmer and I inspected. 3CAH feed around the stem of young plants, girdling the stem near the soil surface. Young seedling plants may lodge from the girdling. When bean pods are set, maturing plants may break over from early seedling damage. Both adult and nymph 3CAH’s will also feed on the petioles of leaves, blooms, and pods. Pod petiole feeding will cause pods to drop to the ground, reducing yield. Soybean plants are most susceptible to main stem girdling when plants are 10 inches or less in height. Once the plant is taller than 10 inches, the main stem is not the preferred feeding site, but the leaf, bloom, and pod petioles may be fed upon. It should be noted that often plants that have been girdled and do not lodge will produce normal yields. The literature reports that 3CAH feeding on leaf petioles and pod stems interferes with photosynthesis and the flow of nutrients to developing seed. Sweep net sampling indicated populations of both insect pests were as high as 3 per sweep. The treatment threshold for grasshoppers once soybeans are in the bloom to pod –fill stage is when plants have 20% defoliation . The pod feeding threshold for grasshoppers in Mississippi is to treat when 50% of plants have one or more pods fed on by grasshoppers. The treatment threshold for 3CAH is one per sweep when plants are more than 10 inches tall.
Treatment decisions for the Tuscaloosa county fields were complicated by the fact that high temperatures had resulted in a significantly reduced pod set to date, even in an irrigated field. (More information about the effects of high temperatures on soybean pod set can be found in the Mississippi Crop Situation Newsletter for July 16, 2010 at http://msucares.com/newsletters/pests/cis/2010/mcs15-10.pdf. After consulting with Dr. Ron Smith the farmer decided to treat the soybeans with the maximum labeled rate of a pyrethroid since many of the grasshoppers were immature and will be much easier to kill now than when they become adults. I had one soybean farmer tell me that one year he could not kill the grasshoppers in one of his northeast Alabama soybean fields and they ruined the crop. The goal of this treatment decision was to hopefully reduce the grasshopper numbers sufficiently to avoid a potential yield loss later and to significantly reduce the numbers of 3CAH’s. Inspection of more soybean fields in Marengo county revealed that a high level of plants (in a field with a 40% stand of soybeans) had sustained a high percentage of girdling when the plants were small. Sweep net sampling broke over many of these girdled plants. The 3CAH population in this field was less than one per sweep and I recommended that the grower wait another week and check the field again before making an insecticide application. The farmers in both Tuscaloosa and Marengo county were reminded that once they treated their soybeans and eliminated the beneficial insect/spider species they faced a greater risk of incurring damaging populations of “worms”.
Mississippi State Extension Entomologist Angus Catchot recently reported that over the last several weeks Mississippi Extension workers have had numerous reports of tobacco budworm moths being flushed in soybeans in the delta region of the state. Ryan Jackson and Clint Allen with USDA-ARS in Stoneville have been collecting populations of bollworms missed with pyrethroids in soybeans. Three populations collected the week of July 25th in the delta behind a pyrethroid application turned out to be 7%, 28%, and 60% tobacco budworms. Tobacco budworms are resistant to pyrethroid insecticides in MS (and in Alabama). Angus stated that although, in some areas of the state budworm numbers may be high in soybeans, most populations are low compared to the bollworm numbers. If high numbers of larvae are left behind a pyrethroid application it is recommended to get a positive ID on the larvae before retreating the field. Two characteristics that can be used to distinguish tobacco budworm larvae from cotton bollworm larvae are depicted in the July 30 Mississippi Crop Situation newsletter which can be viewed at (http://msucares.com/newsletters/pests/cis/2010/mcs17-10.pdf) Some Mississippi farmers have chosen to let remaining worms cycle given the cost of treatment for tobacco budworm. Cotton bollworm (i.e. pod worm) populations have thus far been much lower than normal in Alabama soybeans.
