Monday, August 6, 2012

Soybean Insect Pest Update

Fall armyworms have been reported infesting pastures across Alabama and last week this pest was found feeding on soybeans in Lawrence County in the Town Creek area. These fall armyworms appeared to have moved from a bermudagrass pasture and crossed a wooded area to reach the soybeans. The caterpillars were feeding on the soybean foliage in the portion of the soybean field adjacent to the woods. Since this report from last week, fall armyworms and yellow striped armyworms have been found in many soybean fields in North Alabama at densities as high as 3 to 4 per 15 sweeps. Since theses armyworms have been found across the R2 to R6 soybeans, it is likely that these worms were derived from eggs deposited in the fields by moths. Only a very limited number of soybean fields have been treated strictly for foliage feeders in Alabama thus far---but numbers of green clover worms are increasing in north Alabama and velvetbean caterpillar numbers are increasing in south Alabama as far north as Monroe county. The economic threshold for foliage feeding caterpillars from pod set to maturity is as follows: Prevent greater than 20-percent leaf  loss. Treat prior to 20-percent leaf loss when five to eight foliage feeding caterpillars (soybean loopers, armyworms, green clover worms, velvetbean caterpillars)  0.25 inch long or longer, are present per foot of row.

Treat when you catch an average of 1.5 foliage feeding worms per sweep. Soybean loopers are harder to dislodge with a sweep net and each looper should be counted twice. Each larva eats a high percentage of the total amount of foliage it consumes during the last 4 to 5 days of the larval cycle. If disease is present in the population growers may wish to delay application  for 2 to 3 days, especially if larvae are less than a half inch long and populations are not much above the minimal threshold. Both fungal and viral disease can wipe out a caterpillar infestation within a week under the right conditions. Podworm and stink bug numbers have been below threshold levels thus far in the vast majority of soybean fields.  However soybeans planted behind wheat could see increasing populations of these two pests as we enter the pod-fill stage.  Three-cornered alfalfa hopper (3CAH) numbers are variable (usually less than 2 per sweep) but numbers in most untreated fields are increasing.  Some growers who applied a fungicide to their soybeans have also included an insecticide to reduce 3CAH numbers.  As soybeans mature we will see more fields with multiple insect pest species present.  At times there will be 2 or more species at densities which are just below treatment threshold levels. It can be difficult to make a treatment decision in this situation.  If you need assistance in making a soybean treatment decision you can call me at 256-627-3450.

By: Tim Reed