Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We are starting to see a significant 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 are finding a few fields with cereal leaf beetle (CLB) adult feeding. Beetle adults can be found moving out of untreated small grains and 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 Mid-West, 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.
During the past week, I have been asked the question, “What insect is causing ‘notched leaf edges’ on whorl stage field corn?” In all cases no insect could be found. This symptom is not caused by an insect but is thought to be from a period of rapid growth when the leaves were developing deep in the whorl. For more information, please see the following link: http://www.agry.purdue.edu/ext/corn/news/timeless/NotchedLeaf.html.
Soybeans
We are starting to see an increase in insect/mite activity in a number of fields throughout the state so be sure to scout fields on a weekly basis.
(a) Thrips- Populations of this insect pest have increased in fields throughout the state. They can 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 have also increased and economic populations and leaf damage symptoms can be found in fields in Sussex County. Although populations can start on field edges, we are also finding hot spots of activity in field interiors so be sure to scout the entire field to make a treatment decision. Labeled materials include dimethoate, chlorpyrifos (Lorsban) , Hero ( zeta-cypermethrin + bifenthrin) and bifenthrin (a number of generic products available). All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – including but not limited to combinations with herbicides, number of applications as well as the time between applications.
(c) Potato Leafhopper – We can also find leafhopper populations in seedling stage soybeans. 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 most fields, defoliation is generally still below the economic threshold level. Before bloom, the defoliation threshold in full season soybeans is 30% defoliation. As of end of this week, the predominant defoliators that we are finding are grasshoppers and 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.