Illinois Natural History Survey - University of Illinois

Forage Crops (Integrated Pest Management)

The alfalfa weevil and potato leafhopper are serious alfalfa pests in Illinois as well as other states. Weevil larvae hatch from eggs laid in alfalfa stems and begin to develop in the leaf folds of the growing tips. As they mature and begin to eat more, weevils move to the other leaves and begin to remove much of the leaf surface. Weevils are especially destructive to the first-crop alfalfa. Potato leafhoppers, on the other hand, are normally more damaging to the second and third crops of alfalfa. Leafhoppers feed by piercing and sucking the juices from the plant tissue. This causes plant stunting, yellowing, and leaf drop, greatly reducing the quality of the alfalfa.

To successfully control both of these pests, growers must pay particular attention to the proper timing of control practices whether they be insecticidal or alternative measures. In addition, a successful pest management program must take into consideration the influence of predators, parasites, and pathogens of each pest. Other factors, such as plant height and stage of growth, are important considerations.

Alfalfa weevil larvae feeding on plant.

Management practices that result in a reduced pesticide load to the environment are especially important in alfalfa production. Alfalfa hay must be free of pesticide contamination in order to produce acceptable dairy products. Illinois Natural History Survey entomologists and University of Illinois Extension personnel are cooperating with their counterparts at the University of Missouri to investigate various alternative control practices and management systems for alfalfa weevil control that are not totally dependent on pesticides. In doing so, researchers are determining which life stage of the insect is most vulnerable, and this may not often be the same stage that causes the damage. Because the weevil deposits its eggs in the plant stem during the fall, winter, and early spring, an attack at this stage may reduce the damaging larval population to below the economic threshold.

Investigations are under way to determine the impact of early spring livestock grazing to reduce overwintering egg populations. This nonpesticide practice for weevil control may be successful; however, other factors, such as yield, plant vigor, and stand longevity, must be considered. Scientists are also studying other means of reducing the egg population, such as a late fall harvest after the first frost and winter burning of the alfalfa stems.

Alfalfa weevil adult.

However, these and other practices may have detrimental influences on the predators, parasites, and pathogens that also provide alfalfa weevil control by reducing larval and adult weevil populations. A number of types of tiny wasps attack the life stages and have proven to be successful in helping to reduce population numbers. A fungal pathogen is especially destructive to larvae in certain years when climatic conditions are ideal for development of the pathogen. An ideal management system must consider all the various control tools including the interactions of one with the others. Entomological research of this type coupled with expertise in crop production and extension activities can result in successful alfalfa production and insect control.

Edward J. Armbrust and Stephen J. Roberts, Center for Economic Entomology

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