Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers 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
In areas where cereal leaf beetles were a problem in small grains, we are receiving reports of adults moving from small grains and feeding on corn. Although we do not have any firm thresholds for this insect on corn, as a general guideline controls may be needed on corn for feeding damage if you find an average of 10 beetles per plant and 50% of the plants exhibit feeding damage. I recently was asked about the potential for cereal leaf beetles to vector disease. In the Midwest, it has been reported that the adult beetle is a vector of maize chlorotic mottle virus (MCMV) that causes corn lethal necrosis disease. Thresholds would be much lower if this disease is an issue. To date, I am not aware of this occurring in Delaware; however, be sure to let us know if you find potential problems.
During the last 2 seasons, we have seen what we feel is stinkbug damage to developing corn ears; however, we are still not sure of the extent of this problem in our area. We are currently seeing a few whorl stage no-till fields planted into burned down small grain covers exhibiting typical stinkbug damage – that is stunted and distorted plants. Reports from states to our south are indicating that stinkbug populations are higher in corn again this season. Information from the University of Georgia from 2008, where they have experienced problems, indicates that:
“(a) Corn is most susceptible to stink bug injury during ear formation before tasseling.
(b) Bugs will feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves, usually outward.
(c) Feeding during silking and pollen shed also will kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages blasts individual kernels usually causing them to abort.
(d) Although we have not developed thresholds for our area, the following thresholds are used in the South: 25% infested plants (1 bug per 4 plants) as a threshold during ear elongation to pollen shed and 50% infested plants (1 bug per 2 plants) during the later part of pollen shed and blister/milk stage.
(e) Initially stink bugs tend to be more prevalent on the field edge, so only a perimeter spray may be needed.”
Soybeans
Be sure to sample fields in the seedling stage for bean leaf beetles, grasshoppers and thrips. We are also seeing an increase in green cloverworm activity so be sure to scout soybeans for all of these defoliators. Grasshopper populations are starting to increase and with the predicted hot weather, we could see an explosion in populations, especially in no-till fields. As barley and wheat are harvested and soybeans are planted, these fields will be 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 maybe needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage. Numerous products are labeled for grasshopper control including a number of pyrethroids, dimethoate, Lorsban, Orthene 97 and Sevin XLR. Be sure to check all labels carefully before combining insecticides and herbicides since there are a number of restrictions on the labels.
Continue to watch carefully for spider mites. Early detection and control is needed to achieve spider mite suppression. With the predicted warm weather, we could see an increase in populations. In addition to dimethoate and Lorsban, we now have Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand alone bifenthrin products (not all are labeled so be sure to check the label) available for spider mite control in soybeans. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control.