Agronomic Crop Insects – April 13, 2012

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

Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. Under the current dry weather conditions, you may need to reduce the following thresholds for each insect pest, especially when both insects are present in a field. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75-100 per stem. Although beneficial insects can help to crash aphid populations, the cooler temperatures have slowed their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. For alfalfa weevil, the following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 to 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem; and 17 to 18 inches tall – 2.5 per stem.

Field Corn
As soon plants emerge, be sure to check all fields for cutworm feeding, even if an at-planting insecticide or a Bt corn was used for cutworm control. In Delaware, a number of cutworm species may be present at planting, including claybacked cutworm, dingy cutworm and black cutworm. The dingy and claybacked species overwinter as half-grown larvae in the soil so they can get a “jump” on black cutworms in terms of feeding damage. In Delaware, black cutworm populations result from local populations overwintering as a pupae or mature larvae as well as moths migrating to the area typically in early March. Factors that favor black cutworm outbreaks include late planting, heavy infestations of winter annual weeds before tillage and planting, reduced tillage, and corn grown after soybean. Fields with a combination of these factors are more attractive to black cutworm moths and are likely candidates for egg laying. Young larvae will feed on plants, resulting in small, irregular shaped holes. They generally begin cutting plants at the fourth instar. Regardless of species, as a general guideline, a treatment should considered in 1-2 leaf stage corn if you can find plants with 10% leaf feeding (please follow this link for a picture of leaf feeding — http://bulletin.ipm.illinois.edu/article.php?id=1134) or 3% cut plants.

Small Grains
Aphids and cereal leaf beetles can still be found in fields throughout the state. As indicated last week, be sure to watch for aphids moving into grain heads. The most significant damage occurs when large numbers of aphids feed on the grain head causing shriveled or blasted heads. During heading, check 50 to 100 heads throughout a field. At grain head emergence, a treatment may be necessary once populations exceed 20‐25 per head. With the current dry weather conditions, you may want to reduce this threshold by one third to one-half. In general, cereal leaf beetle populations remain low throughout the state, with a few fields having hot spots of economic levels.

Once grain heads have emerged, you should also begin sampling small grains for grass sawfly and armyworm larvae. Although we see economic damage from local overwintering armyworm populations, we often see significant outbreaks in years when high levels of moths coming from the south migrate to our area. You can look at the following link from Kentucky that compares their moth flights this year to 2006 & 2008 – which they consider outbreak years. (http://www.uky.edu/Ag/IPMPrinceton/counts/taw/tawgraph.htm ). Their first catches of the season where earlier and higher; however, only time will tell if this trend will continue. They are also showing up in wheat fields earlier in the south, so be sure you scout field as soon as heads emerge.

Remember, armyworm larvae are nocturnal so look for larvae at the base of the plants 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. Although armyworms initially feed in the lower canopy on the leaves, under drought conditions we could see them quickly moving up the plant and more quickly clipping the stem just below the heads. Heavy defoliation of the flag leaf can also result in economic loss. In addition, significant stem/head clipping can occur in barley in a short period of time.

As of early this week, we have not found any sawfly larvae in our limited surveys. 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 inner space 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 inner space 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. The higher rates of labeled insecticides are needed for grass sawfly control. (http://ag.udel.edu/extension/IPM/ExtensionFactSheets/SawflyandArmywormIPM-6.pdf)

Timothy
Cereal rust mites remain active in fields that have not been treated, so if you have not checked fields for this pest, be sure to sample all fields. Symptoms can appear as retarded growth, leaf curling, stunting, and plant discoloration. Injured plants appear to be drought stressed even when adequate moisture is available for plant growth. There are no established economic thresholds for the pest; however, treatment is recommended in fields with a previous history of cereal rust mites and/or when 25% of the plant tillers exhibit curled tips of the new leaf blades within several weeks following green-up. The use of a 20x-magnifying lens is often necessary to find mites on leaves. The only effective and labeled material on timothy is Sevin XLR Plus. Be sure to read the label for information on the number of applications per season as well as the days to harvest. For effective rust mite control, the use of the higher labeled rate and at least 25 gal/acre of carrier to get good coverage of leaf surfaces generally results in better control.

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