Agronomic Crop Insects – May 6, 2011

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

Alfalfa
Economic levels continue to be found in fields throughout the state. If economic levels of alfalfa weevil are present before harvest and you cut instead of spray, be sure to check fields within one week of cutting for damage to the regrowth. If temperatures remain cool after cutting, there is often not enough “stubble heat” to control populations with early cutting. In some cases, damage to re-growth can be significant. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady.

Field Corn
In areas were corn is emerged, be sure to sample all fields for cutworms and slugs. For cutworms, fields should be sampled through the 5-leaf stage for damage, even if an at-planting control strategy was used. We are finding leaf feeding by cutworms as well as slug damage in some fields 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% cut plants or 10% leaf feeding. With the continued wet weather, slug damage is starting to increase on newly emerging corn, especially in no-till situations. As indicated in a past newsletter, DuPont issued a 2ee recommendation for Lannate LV for slug management last season; however, we have limited experience with its use (http://www.cdms.net/LDat/ld183004.pdf). Most of our experience has been with the use of Deadline M-Ps which has provided control in past years. Be sure to read the following fact sheet from Ohio for more information on slug management (http://ohioline.osu.edu/ent-fact/pdf/0020.pdf).

Small Grains
Continue to watch for the movement of aphids into grain heads. In many cases, beneficial activity will not be high enough to take care of fields were populations are moving from the lower canopy of the plants into the grain heads. Continue to watch for cereal leaf beetles since larvae can be found in unsprayed fields throughout the state. We can find larger larvae up on the flags leaves and smaller larvae still in the lower plant canopy. Depending on the temperature, newly hatched larvae will feed for up to 3 weeks. Research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage, so continue to sample carefully for this insect. The treatment threshold is 25 eggs and/or small larvae per 100 tillers. Treatment is suggested when the egg threshold is reached and more than 50% of the sample consists of larvae, i.e. 50% egg hatch.

Wheat and barley should also be sampled for sawfly and armyworm larvae. Both can now be found in fields throughout the state; however, population levels and presence vary from field to field so scouting will be important. Armyworm larvae are nocturnal so larvae are generally found at the base of the plants during the day. However, during cool, cloudy weather, you may also see them feeding on the stems during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half.

 

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