Agronomic Crop Insects – May 15, 2009

Joanne Whalen, Extension IPM Specialist;

In addition to checking for weevils feeding on re-growth, be sure to begin checking all fields for leafhoppers within one week of cutting. Spring planted fields should also be sampled 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. For cutworms, fields should be sampled through the 5-leaf stage for damage. We are finding leaf feeding by cutworms as well as slug damage so be sure you do not confuse the damage. If slugs are damaging plants, you will be able to see “slime trails” on the leaves. As a general guideline for cutworms, a treatment should be considered in 1-2 leaf stage corn if you find 3 percent 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.

We are starting to see an increase in slug damage. With the continued cool, wet weather, slug eggs as well as newly hatched juvenile slugs can be found under residue in no-till fields. In past years, liquid nitrogen fertilizer applications have been used to help plants grow ahead of the damage; however, the use of Deadline M-Ps should be considered if the weather remains cool and wet and damage is increasing. A new fact sheet from Ohio State provides good information on slug biology, scouting and management of slugs on field crops

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 plants with larvae less than one-inch long.

Small Grains
Continue to scout fields for cereal leaf beetles, armyworms and sawflies as well as aphids feeding in the heads of small grains. With the extended cool weather, the beneficials may not help to reduce aphid populations. Since populations of all of these insects vary from field to field, fields should be scouted to determine if economic levels are present. As a general guideline, if multiple insects are present, the threshold for each insect should be reduced by 1/3.

With the continued cool, wet weather, seed corn maggot will 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/Trilex 6000 (for use in commercial seed treaters only) and one hopper box material containing permethrin ( Labels state early season protection against injury by seed corn maggot.

As the earliest beans emerge, slugs, bean leaf beetles and grasshoppers can be potential problems. As we found out in 2008, slug damage can be severe on soybeans if slugs are actively feeding when germination occurs since the soybean plant’s growing point is within the cotyledons as they emerge.

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 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. 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/Cruiser MAX and Gaucho/Trilex 6000), they 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 still 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 have been heavy and bean pod mottle virus was found.

Regarding grasshoppers, in general, the treatment threshold is 1 per sweep and 30% defoliation. Multiple applications are often needed for grasshopper control.

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