Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. In addition to adults, we have started to find the first nymphs. The nymphs can cause damage very quickly so sample fields on a weekly basis. Once plants are yellow, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.
Field Corn
Armyworms can be found moving out of small grains and into adjacent field corn. Remember, worms must be less than 1 inch long to achieve effective control. The treatment threshold for armyworms in corn is 25% infested plants with larvae less than one-inch long. Large larvae feeding deep in the whorls will be difficult to control.
Grass Hay Fields
Please note that in addition to Warrior and Mustang MAX, Intrepid also has a label for armyworm control in grass hay fields. Please see the label for use rates and restrictions (http://www.cdms.net/LDat/ld61K032.pdf).
Soybeans
Be sure to sample fields in the seedling stage for bean leaf beetles, grasshoppers and thrips. We continue to see an increase in activity for all three insect pests. Significant grasshopper population pressure can be found in both no-till and conventional soybean fields. As barley is harvested and soybeans are planted, these fields will be especially susceptible to attack and grasshopper feeding can often cause stand loss. If stand reductions are occurring from plant emergence to the second trifoliate, a treatment should be applied. Although no precise thresholds are available, a treatment may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the prebloom stage. As a general guideline, a treatment may be needed for bean leaf beetle if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages. The Iowa State economic threshold for cotyledon stage is four beetles per plant. Once plants reach the V1 and V2 stages, their thresholds increase to 6.2 (V1 stage) and 9.8 (V2 stage) beetles/plant. These treatment thresholds should be reduced if virus is present or you suspected virus the previous season. As far as thrips, information from North Carolina indicates that “soybean thrips and other thrips species can feed and reproduce on the leaves and buds of soybean seedlings. Their feeding creates bleached-out lesions along the leaf veins and gives a silvery/bronzed appearance to the leaf surface when damage is severe. These insects are very small (less than 1/10 inch) and are torpedo shaped. While thrips always occur on soybean seedlings, it is only during outbreak years that they cause concern. In particular, during dry weather and on earlier planted full-season soybeans, thrips populations can explode when plants are growing slowly. Under these circumstances thrips injury will occasionally kill seedlings. Other stressors, such as nutrient deficiencies and herbicide injury, can add to thrips damage and cause plant loss.” Yellowing can occur from thrips but there are also a number of other factors that can cause yellowing so it is important to scout fields to identify what is causing the yellowing. Although no precise thresholds are available, as a general guideline, treatment may be needed if you find 4-8 thrips per leaflet and plant damage is observed.
In addition to these 3 insect pests, you should also start looking carefully for spider mites. There were reports of spider mites on volunteer soybeans before the rains and unfortunately they have probably survived. Early detection and control is needed to achieve spider mite suppression. In addition to dimethoate and Lorsban, we now have Hero labeled on soybeans. The bifenthrin component in this mix is the material that will provide spider mite suppression. However, to be effective it should be applied before mites explode. Please refer to the label for use rates and restrictions – you will need the high rate for spider mite control. It should also be noted that the label states do not make applications less than 30 days apart (http://www.cdms.net/LDat/ld80Q005.pdf).