Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
In addition to checking for weevils feeding on re-growth, be sure to check fields for leafhoppers within one week of cutting. You will also need to carefully sample all spring planted fields since they are very susceptible to damage. Once the damage is found, 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
Continue to sample all fields for cutworms, slugs and true armyworm. Be sure to sample all fields through the 5-leaf stage for cutworm damage. As a general guideline, a treatment should be considered if you find 3% cut plants or 10% leaf feeding. If cutworms are feeding below the soil surface, it will be important to treat as late in the day as possible, direct sprays to the base of the plants and use at least 30 gallons of water per acre. You should also sample no-till fields for true armyworms, especially where a grass cover or volunteer small grains were burned down at planting. As a general guideline, a treatment may be needed for armyworms if 25% of the plants are infested with larvae less than one inch long. As small grains dry down, you should also watch for armyworms moving from small grains into adjacent corn fields.
Small Grains
Continue to scout fields for armyworms and sawflies, as well as aphids feeding in the heads of small grains. We are starting to see an increase in the number of armyworm larvae found in untreated wheat and barley fields. The following are a few things to remember about the biology of armyworms and the damage they can cause in small grains:
● Young larvae (less than ½ inch long) generally feed on the upper leaf surface.
● Larger larvae can feed heavily on the leaf blades and weeds.
● The last instar (1.5 inches long and greater) will consume 80 percent of all the plant material eaten during their larval development. This stage lasts six to eight days before moving into the soil to pupate.
● Heavy defoliation of the flag level can result in significant economic loss.
● Unlike the sawfly, 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. They also can clip the heads of barley faster than wheat.
● Larvae can feed on the kernel tips of wheat, resulting in premature ripening and lower test weight. If fields were sprayed early, be sure to check fields to be sure that you do not miss an infestation.
Soybeans
Throughout the month of May, seed corn maggot will continue to be a potential problem in no-till soybeans as well as conventional soybeans where a cover crop is plowed under before planting or where manure was applied. All of these situations are attractive to egg laying flies. Control options are limited to the commercial applied seed treatments, Cruiser/Cruiser MAXX and Gaucho (for use in commercial seed treaters only) and one hopper box material containing permethrin (http://www.tracechemicals.com/trace/labels/KernelGuardSupremelabel.pdf ). Labels state early season protection against injury by seed corn maggot.
As the earliest beans emerge, be sure to watch for slugs, bean leaf beetles and grasshoppers. We have seen all three pests feeding on seedling stage beans.
On the earliest emerged fields, be sure to watch for bean leaf beetle adults feeding on the cotyledons and first true leaves. In recent years, bean leaf beetle populations have been heavier in the Mid-Atlantic and we can find damage on the earliest planted beans. Damage appears as scooped out pits on the cotyledons and leaf feeding appears as distinctive, almost circular holes, which are scattered over the leaf. Refer to the following link for pictures of adults and damage (http://www.ent.iastate.edu/imagegal/coleoptera/beanlb/). Even though the feeding by first-generation beetles on soybean leaves has seldom resulted in economic yield losses (except if virus is vectored), fields should be scouted carefully to assess the damage. In the Midwest, this beetle vectors bean pod mottle virus. The presence of bean pod mottle virus was confirmed for the first time in Delaware in 2007 by Bob Mulrooney. The second-generation feeding on pods in late summer could cause significant damage. This generation would also be the generation to vector virus next spring. There are numerous treatment guidelines available. However, as a general guideline, a treatment may be needed if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages. The Iowa State economic threshold for cotyledon stage is four beetles per plant. Once plants reach the V1 and V2 stages, their thresholds increase to 6.2 (V1 stage) and 9.8 (V2 stage) beetles/plant. These treatment thresholds should be reduced if virus is present or you suspected virus the previous season.
As far as the commercial applied seed treatments (Cruiser and Gaucho), both materials are labeled to provide early protection against injury from bean leaf beetle. However, these seed treatments will not limit later population growth in mid to late summer. For growers who choose to control overwintering bean leaf beetles to limit virus transmission, information from the Midwest indicated that an early season foliar spray after plant emergence, followed by a second spray in July for the first generation beetles might be tried. Because seed treatments will offer control of the overwintered beetles and reduce feeding injury, growers might want to use seed treatments to replace the early season foliar spray. Currently, we do not have all the answers as to whether controlling the overwintered beetles with seed treatments will reduce virus transmission. Data from the Midwest is variable – some say that the use of seed treatments may be one part of an overall effective pest management program, while other data suggests that this approach might not give economic control of the virus. We are again evaluating seed treatments this year in areas of the state where bean leaf beetle populations were high in 2007 and bean pod mottle virus was found.
Small grasshoppers can be found in full season no-till plantings. In general, the treatment threshold for grasshoppers is 1 per sweep and 30% defoliation. Early detection of small grasshoppers and multiple applications are often needed for grasshopper control.