Agronomic Crop Insects – May 14, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
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
Be sure 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.

With the recent cooler, wet weather in some areas of the state, we are starting to see an increase in slug damage. Newly hatched juvenile slugs can be found under residue in no-till fields. The use of Deadline M-Ps (or other available metaldehyde baits) should be considered if the weather remains cool and wet and damage is increasing.

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.

We have also seen an increase in bird damage that is sometimes confused with cutworm damage. You can distinguish bird damage from cutworm damage by the pattern in the field. Generally longer strips of damaged plants, plants pulled out of the ground, and/or plants cut high that are compressed at the base of the stems, all indicate bird damage. Although birds can cut plants off at the soil surface, they tend to pull plants out of the ground. In addition, if you look closely you will see “bird prints” near the missing plants or holes where birds have pulled plants out of the ground.

Lastly, there are a number of new transgenic corn traits that should be available to producers in 2011. The following link from Ron Hammond in Ohio which provides a good summary of these new traits: http://corn.osu.edu/newsletters/2010/2010-12/new-transgenic-corn-products

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 recent cooler weather, the beneficials may not help to reduce aphid populations. Also, with the recent weather patterns, the hatch of armyworm larvae will be staggered –i.e. there will be large and small larvae in fields. It is important that you scout fields on a weekly basis until harvest for armyworm and sawfly larvae. Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley. 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 one third.

Soybeans
In full season no-till soybeans as well as in conventional fields where a cover is worked under before planting, seed corn maggot can be a potential problem. These situations are attractive to egg laying flies. Control options are limited to the commercial applied seed treatments (Cruiser/Cruiser MAXX, Gaucho/Trilex 6000 and Innovate) 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.

In recent years, slugs have also been a significant problem in no-till soybean fields. We can find small slugs that have recently hatched under surface residue. 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. If slugs are actively feeding when germination occurs, they can feed on the cotyledons and cause death of the soybean plant. Therefore, compared to corn, it can be difficult to time an application of a bait treatment. Fields should be scouted and tillage considered if you can easily find slugs under the surface trash and weather remains cool and wet. This option has worked in recent years but only if there is a period of time between tillage and planting. In addition, if slug numbers are high, it may not get enough of them but overall it should help. Another option might be planting later into warm soils to promote rapid early growth and help young plants outgrow the slug pressure. Delayed planting can help if the weather turns warm and dry; however, this option was not very effective during the 2009 season since the weather remained cool and wet throughout the summer. Unfortunately, we still have a lot to learn about slug management in soybeans.

As the earliest beans emerge, you should also watch for bean leaf beetles and grasshoppers. Bean leaf beetle adults feed 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 leaf feeding by first-generation beetles on soybean leaves has seldom resulted in economic yield losses, fields should be scouted carefully to assess the damage. In areas of the state where bean pod mottle has been identified, it is the overwintering beetle that vectors the virus. The second-generation feeding on pods in late summer can cause significant damage. 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, Gaucho/Trilex 6000 and Innovate), 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.

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

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