Agronomic Crop Insects

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

Alfalfa
If economic levels of alfalfa weevil are present before harvest and you decide to 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.

Small Grains
With the recent warmer temperatures, we saw an increase in true armyworm catches in the Harrington, Little Creek, Rising Sun and Greenwood areas. In general, these numbers are lower compared to this time last year. Many factors, particularly disease, predation and parasitism can impact how well the eggs and caterpillars survive. Although true armyworms overwinter in our area, we can also get migrant moths from the South. Therefore, be sure to scout all small grains for armyworms. Although the combination sprays of fungicides and insecticides have worked in the past to control armyworms, it will still be important to re-check fields after application to be sure you have gotten control.

In addition to armyworms, do not forget to watch for sawflies, cereal leaf beetles and aphids. We have seen an increase in cereal leaf beetle egg laying. We have also heard reports of increased levels of aphids in barley fields; however, in many cases the aphids are still in the lower plant canopy. Since aphids feeding in the heads of small grains can result in a loss in test weight, be sure to watch for movement of aphids into the grain heads. As a general guideline, a treatment should be considered if you find 20 aphids per head and beneficial insect activity is low. You need at least one beneficial insect per every 50-100 aphids to help crash populations. Be sure to check the days between last application and harvest when selecting a spray material.

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