Insects: Alfalfa - new seeding

Part 2, Section 6: Forages Pest Management

Forages Pest Management

INSECTS

Alfalfa - New seeding

Potato leafhoppers are the most destructive insect pest species on new seedings of alfalfa in the state. Stress to alfalfa seedlings caused by this pest species can affect the vigor and later performance of the plants and can influence stand longevity. To obtain optimum stands, yields, and quality, it is essential to use sound control measures for potato leafhopper on new seedings.

In most years, leafhopper populations are high enough in some fields to cause appreciable losses to newly established stands. Two methods of reducing this damage exist, seeding glandular-haired, leafhopper-resistant alfalfa varieties and monitoring leafhopper populations by periodic sweeping followed by insecticide applications.

When alfalfa is typically seeded in spring, there are no insect pest species that influence yield, with the possible exception of cutworms. Clover root curculio do not lay eggs in spring-seeded alfalfa, because they already have moved into established stands when new seedings are seeded. The same is true for alfalfa weevil, which migrate into established stands to lay their eggs in late March and early April.

Regrowth after the first cut must be monitored closely for insects, starting when the regrowth is 2 to 3 inches tall. See Tables 2.6-10 and 2.6-11 for materials and rates.

Currently, malathion is the only control method for potato leafhoppers on seedling alfalfa established in spring grains such as oats. This compound has a very short residual, however, and application requires driving over the alfalfa and small grain.

Glandular-haired, potato leafhopper-resistant alfalfa is a newer pest management tool for farmers. This technology provides season-long protection against the pest. These varieties provide resistance through glandular hairs, which produce a sticky substance that prevents the nymphs from feeding and may contain a toxin. Expression of these hairs, however, is increased after the first cutting. It also should be noted that not all plants express the glandular hairs. In the newest varieties, approximately 70 to 80 percent of plants have the hairs. The remaining plants are still susceptible to injury. Older varieties express the glandular hairs on only between 30 and 50 percent of the plants. Therefore, although the first cutting of a new seeding is most susceptible to injury, some crop injury can occur during any cutting when leafhopper pressure is high. In some situations, it still may pay to apply an insecticide for the pest. During the new seeding year, it would be best to continue to use the regular thresholds resistant alfalfa. Following the first year, the thresholds can be tripled to three times the normal thresholds (see next section).

The economics of using these varieties is improving each year, as the percentage of plants expressing the glandular hairs increases, and other agronomic characteristics of the plant improve. This technology is a good alternative for farmers who typically do not treat for leafhopper or who would like to organically produce the crop. It should be recognized that this technology does not provide protection against alfalfa weevil or other alfalfa pests.