Agronomic Crop Insects – June 5, 2009

Joanne Whalen, Extension IPM Specialist;

Continue to sample for potato leafhoppers on a weekly basis. Although adults are the main life stage present, we will soon see the first nymphs. Although both life stages can damage alfalfa, the nymphs can cause damage very quickly. 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.

Small Grains
We have received numerous reports and have seen a number of fields this week with high levels of armyworms in both wheat and barley that did not receive an earlier insecticide treatment. In some barley fields, significant head clipping has already occurred. As indicated in past newsletters, damage can quickly occur in barley. In many cases there is a mixture of worm sizes so the potential for head clipping is high, especially in barley. Be sure to scout all wheat and barley fields carefully and watch for head clipping. Since we are close to harvest for barley, the only control option at this point is Lannate which has a 7 day pre-harvest interval (PHI) between application and harvest. As a reminder, Warrior II and generic lambda-cyhalothrins products have a 30 day PHI between application and harvest. Be sure to read the label before applying any insecticide for the rate, days between last application and harvest and other restrictions.

Continue to sample for bean leaf beetles and grasshoppers. In the earliest planted and emerged fields, we have started to see an increase in activity for both insects. 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 pre-bloom 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.

With the continued cool, wet weather, we continue to hear reports and have seen no-till soybean fields with significant slug damage. Although tillage will probably help and we have seen where it has at least helped to get soybeans out of the ground and growing this year, it may not be the total answer if populations are high and the weather does not turn around after planting. We have seen significant damage this spring in conventional corn fields where a cover was tilled under and then the field immediately planted. The only labeled, effective option for control is the use of the Deadline M-Ps. Be sure to read the new fact sheet from Ron Hammond from Ohio that provides excellent information on slug biology and management

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