Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. Both life stages can damage alfalfa but the nymphs can cause damage very quickly. 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.
If you have planted a glandular haired variety, we do not have any local data but here is some information from Ohio State regarding treatment thresholds on these varieties:
“If the alfalfa is one of the glandular-haired, leafhopper-resistant varieties of alfalfa, the economic threshold is three leafhoppers per inch of growth (24 leafhoppers for 8” tall alfalfa, for example). However, if the resistant alfalfa is a new planting this spring, growers should use thresholds meant for regular alfalfa during the first growth from seeding. Because resistance improves as the seedling stand develops, research suggests that the threshold for a resistant variety can be increased after the first cutting.”
Field Corn
Economic levels of cutworms continue to be found in fields that did not receive a treatment at planting. Fields planted next to barley should be scouted for armyworms moving from barley into adjacent corn fields. Control will be difficult once larvae move deep in whorls. Remember, worms must be less than 1 inch long to achieve effective control, and some labels indicate that larvae need to be even smaller. The treatment threshold for true armyworms in corn is 25% infested plants with larvae less than one-inch long. We are just starting to survey corn for stink bugs and can find signs of some seedling stage damage. In areas of North Carolina, they are already reporting pretty heavy stink bug pressure on seedling field corn. Please see read the following article by Dr. Dominic Reisig at the NCSU station on Plymouth for details https://entomology.ces.ncsu.edu/2016/05/how-to-avoid-a-stink-bug-disaster-in-corn-2/
Small Grains
Continue to watch for true armyworms in wheat and barley that has not been treated. We are hearing reports of economic damage in fields on the eastern shore of Virginia. Armyworms cause two types of damage – leaf feeding and head clipping. Although leaf feeding is rarely extensive enough to cause damage, heavy defoliation of the flag level can result in economic loss. Armyworms generally begin head clipping when all vegetation is consumed and the last succulent part of the plant is the stem just below the grain head. Larvae can feed on the kernel tips of wheat, resulting in premature ripening and lower test weight. On barley, significant stem clipping can occur in a short time. As far as cereal leaf beetle, research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage. Before making an application of an insecticide, be sure to check all labels for the number of days between last application and harvest. Most of the labeled pyrethroids have a 30 day pre-harvest interval (PHI) restriction. One exception is Mustang Maxx which has a 14 day pre-harvest interval (PHI) restriction.
Similar to North Carolina, we are also finding higher levels of native brown stink bugs in wheat fields throughout the state. Although treatment for stinkbugs is generally not needed in wheat (research from the south shows it takes high numbers to damage wheat), it will be important to scout wheat before harvest to watch for the potential movement from wheat into field corn. The following links from North Carolina provide information on stinkbug monitoring and management in wheat and corn
https://entomology.ces.ncsu.edu/2015/05/stink-bugs-in-eastern-north-carolina-wheat/
https://entomology.ces.ncsu.edu/2016/05/how-to-avoid-a-stink-bug-disaster-in-corn-2/
Soybeans
Be sure to sample fields starting at early emergence for slugs, bean leaf beetles and grasshoppers. Unfortunately, control of slugs in soybeans is very difficult to time, especially when slugs have already hatched, because the plant’s growing point is within the emerging cotyledons. We are still working on how to effectively time bait applications in soybeans. Applications applied so far this season have not resulted in satisfactory control. If germination occurs when slugs are actively feeding, the slug can feed on the cotyledons and feeding point that can result in the death of the plant. Although tillage before planting is not the total answer, it can help if conditions are dry after tillage and beans are able to quickly jump out of the ground. Good seed slot closure will also be critical since significant stand reductions can occur when slugs are feeding on seeds below the ground preventing plant emergence. For bean leaf beetles and grasshoppers, 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 may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage. As a general guideline, a treatment may be needed for bean leaf beetle if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages.