Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to see a few fields with yellowing. 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
Over the past week, we have seen an increase in Japanese beetle adults feeding on the leaves of field corn. Population levels are higher this year and it is thought to be a result of the wet weather last season. In general, leaf feeding from Japanese beetles has rarely caused economic loss in field corn and there are no thresholds for Japanese beetle defoliation. The next question I have received is 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. Japanese beetles generally 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 comes from the Midwest regarding when Japanese Beetles are most likely to cause damage. 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 neededif silks are clipped back to less than 1/2 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).
Native brown stink bug populations are also starting to increase in field corn, we have not observed any Brown Marmorated stink bugs, so you should also be watching for stink bugs in corn. As a review, corn is most susceptible to stink bug injury during ear formation before the tassel stage (VT). Bugs feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves, usually outward. Feeding during silking and pollen shed (R1) will also kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages can blast individual kernels usually causing them to abort. Although we do not have any thresholds for our area, the following thresholds from the South can be used as a guideline for treatment;
(a) When the ear is forming, during ear elongation, and during pollen shed, the treatment threshold is one stink bug per four plants (25% infested plants).
(b) From the end of pollen shed to blister/milk stage, the threshold is one stink bug for every two plants (50% infested plants).
Soybeans
Continue to watch for spider mites, thrips and defoliators (green cloverworm, bean leaf beetles, grass hoppers, etc.) as indicated in last week’s newsletter. 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. Before bloom in full season soybeans, the defoliation threshold should be used to make a decision on when to treat for defoliators (green cloverworm, Japanese beetles, grasshoppers, bean leaf beetles, etc.). The threshold on soybeans pre-bloom is 30% defoliation. There are also reports from the South of high fall armyworm populations in soybeans (with true armyworm in the mix). Since fall armyworm is a migratory pest, you will want to watch for this insect showing up earlier than normal in soybeans.