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
Be sure to sample for stink bugs, Japanese and Oriental Beetles and grasshoppers in field corn. We find all of these insects in fields throughout the state at varying levels.
(a) Japanese Beetles, Oriental Beetles and Others in the Same Family: During the last two years, we have observed more Japanese beetle adults present in silking corn. Over the last week, we have seen an increase in Oriental Beetles, Japanese Beetles and other look-a-like beetles in the same family. In some areas of the state, Japanese beetles and related beetles can be found feeding on the leaves. In general, leaf feeding from Japanese beetles has rarely caused economic loss and there are no thresholds for Japanese beetle defoliation. I am often asked if these beetles will move to the ears and clip the silks. The answer is yes it is possible and we have seen silk clipping in past years. These beetles stay around longer in corn because they are attracted to the silks. In many years infestations are spotty and can be confined to field edges so, once again, scouting the entire field is needed to make a treatment decision. All of the information we have regarding when Japanese Beetles are most likely to cause damage comes from the Midwest . The following link to a fact sheet from Purdue provides good information on scouting and decision making. There is also an IPM tip at the end from Bob Nielsen about how to determine what percent of the pollen has been released.
http://extension.entm.purdue.edu/fieldcropsipm/insects/corn-japanese-beetles.php.
As a general rule, treatment for Japanese beetle may be needed if silks are clipped back to less than half an inch when less than 50% of the plants have been pollinated and Japanese beetles are still present and actively feeding. Pollen shed for an individual tassel generally takes 2-7 days to complete and 1-2 weeks for an entire field (information from Bob Nielson, Purdue University).
(b) Grasshoppers: During the past week, we have observed an increase in grasshoppers feeding on corn. Nymphs and adults will feed on corn in any plant growth stage. In most years, they are not observed until R1 but this year they can be found on earlier stages of corn. In general, the outer rows of corn are usually the first attacked, but as the grasshoppers reach the adult stage they move further into the field eating the leaves, silks, and ear tips. Although we do not have thresholds available in our area, the following link provides information on thresholds used in the Midwest
https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-grasshopper.php.
(c) Stink Bugs: Be sure to watch for stink bugs moving from wheat and/or weedy areas into corn before tassel emergence. Information from Georgia indicates that corn is most sensitive to stinkbug damage during ear formation before silking. Stink bug feeding damage to small developing ears starting at V12 (vegetative tassel stage) can deform ears into a C or boomerang shape. These ears fail to develop properly and may be more susceptible to infection by diseases. Additional information from North Carolina on sampling and treatment timing can be found at sampling and treatment timing in field corn form North Carolina
https://entomology.ces.ncsu.edu/2016/06/stink-bugs-moving-into-corn/
Soybeans
Continue to watch for spider mites, thrips, potato leafhoppers, and defoliators (green cloverworm, bean leaf beetles, grass hoppers, etc.). Another group of defoliators, the Japanese beetle, and other look-a-like beetles all in the same insect family, are also starting to show up in soybeans. One defoliator that has been present in more fields over the past two seasons and is showing up again this year is the silver spotted skipper. This insect caused significant losses in the 1980s when its host range shifted from leguminous shrubs and trees to soybeans. We have seen an occasional field with economic levels of defoliation from this insect in recent years so it should also be considered when sampling for defoliators. For more information and pictures of the silver spotted skipper please visit the following link:
http://cropwatch.unl.edu/archive/-/asset_publisher/VHeSpfv0Agju/content/silver-spotted-skipper-on-soybeans