Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We are starting to see an increase in populations and 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
We continue to see cereal leaf beetle (CLB) adult feeding on the edges of fields. These beetle adults moved out of untreated small grains and are feeding on the edge of corn fields. Although we do not have any firm thresholds for this insect on corn, as a general guideline controls may be needed if you find an average of 10 beetles per plant and 50% of the plants exhibit feeding damage. 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 for beetle feeding would be much lower if this disease is an issue. We have not seen this virus in Delaware corn fields; however, please let us know if you suspect a problem.
We are also seeing the first emergence of Japanese beetle adults. In addition, a few native brown stink bugs can be found in corn fields.
(a) Japanese Beetles: As you may recall, we did see more Japanese beetle adults in silking corn in 2014. This was thought to be the result of the wet weather in 2013. We also saw more damage from grubs this spring resulting from an overwintering population. So that means that adult population levels may be higher again this year. In some years we see Japanese beetles 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 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 needed if 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).
(b) Native Brown Stink Bugs – We are also seeing the first native brown stink bugs in field corn but we have not observed any Brown Marmorated stink bugs. 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 used in the South is one stink bug for every two plants (50% infested plants). Please refer to the following link for more information on stink bug management in field corn http://entomology.ces.ncsu.edu/2014/07/stink-bugs-in-corn/.
Soybeans
Be sure to watch for an increase in insect and mite activity. So far, insect and mite activity has been generally low but this can change quickly. In our statewide soybean surveys this past week, the most common insects found have been thrips, potato leafhoppers and green cloverworm.
(a) Thrips- Thrips 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 are often found on seedling stage soybeans, it is generally 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.
(b) Spider Mites – Populations are still low in most fields but we have found isolated patches of mites in fields in Kent and Sussex Counties.
(c) Potato Leafhopper – We can also find leafhopper populations in soybeans in the V-1 to V-4 stages. As a general guideline, a control may be needed for leafhoppers if you see plant damage and you find 4 leafhoppers per sweep in stressed fields and 8 per sweep in non-stressed fields.
(d) Defoliators – Although a variety of defoliators can be found in fields throughout the state, defoliation is generally still below the economic threshold level. Before bloom, the defoliation threshold in full season soybeans is 30% defoliation. Once fields reach the bloom stage, this threshold decreases to 15% defoliation. As of end of this week, the predominant defoliator that we are finding is the green cloverworm larvae. Green cloverworm are light green with three pairs of white stripes running the length of the body. In addition to the three pairs of legs near the head, they have three pairs of fleshy legs near the middle of the body, and one additional pair at the end of the body. Larvae wiggle vigorously when disturbed. Smaller larvae may drop from the leaf when disturbed. Young larvae skeletonize the underside of the leaf. Older larvae chew irregular shaped holes in the leaves and can eat all of the leaf except large veins. Although populations of green cloverworm generally increase in number from July through September, if the weather turns dry, we have seen an earlier increase in numbers.