EXTENSION 
INSTITUTE OF AGRICULTURE AND NATURAL RESOURCES         
UNIVERSITY OF NEBRASKA-LINCOLN 
NORTHEAST RESEARCH & EXTENSION CENTER       
601 EAST BENJAMIN AVENUE, SUITE 104
NORFOLK, NE 68701-0812
PHONE: (402) 370-4000 
FAX (402) 370-4010

Cowpea Aphids

Cowpea aphids on alfalfa have been reported in Cuming County . The grower has harvested and should wait to see if they return. With previous findings over the years in Knox and Dixon counties, it means that they can be present anywhere in northeast Nebraska . Be on the lookout for these pests in alfalfa.

The cowpea aphid is easily distinguished from other aphids in alfalfa largely because it is the only black aphid found infesting the crop. In general, it is a relatively small aphid, less than 2 mm long. Non-winged and winged adults are usually shiny black, while the smaller nymphs may appear to be a dull gray to black. The first half of the antennae is white, and the legs are usually a creamy white color with blackish tips. In alfalfa, these aphids prefer to feed on young terminal growth, but can be found infesting leaves, blooms, and stems. Damage symptoms include yellowing, wilting, and dieback. In general, legumes can be seriously damaged, either by direct insect feeding or by the transmission of virus diseases. This aphid produces a considerable amount of honeydew upon which sooty mold grows. The honeydew also makes the alfalfa sticky, which causes problems with harvest.

Threshold suggestions for cowpea aphid control are similar to those of blue alfalfa aphid: 10-12 per plant in alfalfa under 10 inches tall or 50 per sweep of a net, 40-50 per plant or 200 per sweep in alfalfa over 10 inches. Insecticide tests at the Haskell Ag Lab showed good control with Mustang Max, Warrior, and Lorsban. (KJ)

Soybean Aphids

Soybean aphids continue to increase slightly in some area fields. It will be a week or so before we see what the hot weather will do to the aphids. While the weather may slow down reproduction, it is by no means a guarantee that they will die out. Remember that in Nebraska they tend to peak in mid-August. Keep scouting. (KJ)

Western Bean Cutworm

Western bean cutworms are still being caught in our light trap. While most of the corn is past the pretassel stage there will still be some egg laying. Most fields with economic levels have been treated by now. (KJ)

Use Test Strips, Field Samples To Evaluate Corn Rootworm Control

Western corn rootworm beetles have been emerging since late June in southeastern and south central Nebraska , and are emerging in the northeast, indicating that rootworm larval feeding is ending. Mid to late July would be a good time to dig roots to evaluate the efficacy of your rootworm management program.

Adult western (left) and northern corn rootworms .(Jim Kalisch, UNL Entomology)

The presence of adult beetles or rootworms in a field is not necessarily an indication of product failure. Soil insecticides are applied in a narrow band or infurrow to the soil, or as a seed treatment, and corn roots grow beyond the treated zone where rootworm larvae may survive. Some rootworm beetles will emerge from Bt corn hybrids labeled for corn rootworm control. Also, plant lodging may occur without significant rootworm feeding. Dig and wash some roots to check for rootworm injury before assuming that rootworm damage is responsible for lodging.

Rootworm efficacy can only be evaluated reliably if replicated, untreated check strips are left in the same field as the treatment. Without check strips, you won't know whether the absence of injury is due to product efficacy or the absence of rootworms. Before corn plants can be rated for injury they need to be at a growth stage where at least three nodes of roots are clearly visible. Dig at least 10 randomly selected plants from several areas of a field. Leave a 9-inch cube of soil surrounding the root system, wash the roots to remove soil and rate each plant for injury using the rating scale. If several weeks have passed between the end of rootworm injury and the time of root rating, new root growth may hide the injury. Examine roots carefully to accurately rate them.

The most widely used method to evaluate root injury has been developed at Iowa State University . It is based on a 0-3 scale. This system was developed to avoid some of the perceived problems with the traditional 1-6 scale, including that the 1-6 scale is not linear (e.g., a rating of 4 does not represent twice as much injury as a rating of 2), and that the 1-6 scale is difficult to explain. The 0-3 scale is linear and the meaning of the injury values are easy to understand. Another potential advantage to the 0-3 scale is that it is more sensitive in detecting differences at low levels of injury compared to the 1-6 scale. This is particularly important in some research applications.

In this scale 0 = no damage, 1 = one complete node of roots is pruned (as defined above), 2 = two complete nodes of roots are pruned, and 3 = 3 nodes of roots are pruned. Fractional ratings are possible, e.g. 1.5 = equivalent of 1.5 nodes of roots pruned. A description of the 0-3 rating system is available on an Iowa State University Web site.

The relationship between root injury rating and yield loss is complex, but usually a root injury rating of 0.25 or more on the 0-3 scale is needed to cause economic yield loss. The corn plant has the capacity to regrow roots and compensate for some early season injury, especially if soil moisture and fertility are adequate during the regrowth period. Bob Wright, Extension Entomologist, Lincoln