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Topics : Species and Forage Variety Trials : Species Fact Sheets : Birdsfoot Trefoil

Species: Birdsfoot Trefoil

Additional birdsfoot trefoil information via the Forage Information System (FIS)


Managing Legume Insects

Management of forage insect pests is aimed primarily at the alfalfa weevil and the potato leafhopper in alfalfa. Other insect pests of forages are minor in comparison and must be dealt with on a field-by-field basis. Economic injury levels for the potato leafhopper and the alfalfa weevil are fairly well established. The economic injury level is the value of crop loss caused by the pests that is equal to the cost of a spray application. Thus, unless the value for the crop loss caused by the pests exceeds the cost of controlling them, it is not profitable to spray. On the other hand, some insect losses could have been avoided if a spray had been applied at the proper time.

A good pest management program requires proper identification of the pest species causing the damage, and determination if the economic threshold has been exceeded. How can you determine if and when a spray application will be profitable? Get a copy of the publication, A Pest Management Program for Alfalfa in Pennsylvania, from your county extension office. This publication will aid you in deciding when it is profitable to control potato leafhopper and alfalfa weevil.

Alfalfa blotch leafminer can be found in practically all alfalfa fields in the state. However, damage will always appear worse than it actually is. The second cutting is usually most severely infested. Control may be justifiable if 30% or more of the leaflets have a mine present.

The adult fly is about .13 inch long and resembles a common housefly. The larvae (maggots) are pale yellow, soft-bodied, shortened, and thickened. At least three generations per year occur in Pennsylvania. Adult females emerge in the spring, cut shallow holes through the lower leaf surface, and deposit eggs under the leaf epidermis. A female lays one to three eggs per leaflet. To feed, the female cuts a hole in the leaf with her oviposter and laps up exposed sap and tissue, forming conspicuous pinholes in the leaves. After the eggs hatch, the larvae tunnel within the leaf, feeding on leaf tissue. The larval stage lasts approximately two weeks. Larval mining causes conspicuous white blotches on the leaflets, which are typically comma-shaped. Blotches and punctures can cause deterioration of foliage quality, loss of photosynthetic area, and defoliation.

Alfalfa weevil is primarily a problem in the first cutting of alfalfa in April and May. Larvae feed within the growing tips, on the upper leaves as they open, and later on the lower leaves. Plants become skeletonized from weevil feeding and the leaves dry, giving the field a frosted appearance. After cutting, the larvae may feed on the new emerging shoots, severely retarding alfalfa regrowth. Adults also feed on the alfalfa plant. Conditions that favor pest development are excessive pesticide use (which destroys biological control agents), mild winters, and warm dry spring weather. Several species of parasitic wasp and a fungus help maintain alfalfa weevil populations.

Black cutworms can cause extensive damage to new seedings in late May early June.

Pea aphid control is sometimes needed, but natural controls are usually all that is needed to keep aphid populations in check. Control may be warranted if populations reach 30 aphids per sweep of an insect net.

Pea aphids are small, green, long-legged insects about .19 inch long. They can be winged or wingless. Like other aphids, the pea aphid damages the plant by removing sap with its sucking mouth parts and possibly by poisoning the plant.

The insect overwinters on alfalfa, clovers, and other perennial plants in either the egg stage or as adult females. In the spring, populations increase on the winter host and begin migrating to other hosts about May 1. Winged females start colonies on new plants by giving birth to live young, which are ready to reproduce in 12 days. A female commonly produces 6 to 7 young per day. There are 7 to 20 generations per year. Pea aphids may be found in forage fields during June and July.

Potato leafhoppers are the most destructive insect pest on new seedings of alfalfa in the state. Stress to alfalfa seedlings caused by this pest can affect the vigor and later performance of the plants and influence stand longevity. New spring seedings are especially vulnerable to attack by leafhoppers. Regrowth of second and third cuttings of established stands is also frequently damaged.

It is essential to use sound control measures for this pest on new seedings in order to obtain optimum stands, yields, and quality. In most years, leafhopper populations are high enough in some fields to cause appreciable losses to newly established stands.

There are several ways to reduce leafhopper damage to the first cutting of new seedings: (1) leafhopper populations can be monitored by periodical sweepings and applying an insecticide accordingly; (2) Furadan (carbofuran) 4F can be broadcast and incorporated prior to seeding; (3) Lorsban (Chloropyrifos) 4E can be broadcast and incorporated prior to seeding. Furadan 4F may be used at seeding time by mixing 2 to 4 pints of Furadan 4F in 15 to 40 gallons of water and spraying on the soil surface; then incorporating. If a herbicide is being used, mix it with Furadan 4F and apply both materials in the same operation. Lorsban 4E can be broadcast and incorporated prior to seeding.

At the time alfalfa is typically seeded in the spring there are no insect pests that will influence yield, with the possible exception of cutworms. Clover root curculio do not lay eggs in spring-seeded alfalfa, because they have already moved into established stands when new seedings are seeded. The same is true of alfalfa weevil, which migrate into established stands to lay their eggs in late March and early April. Furadan and Lorsban cannot be expected to provide adequate leafhopper control on alfalfa seeded before early May.

The effects of Furadan and Lorsban will not last much longer than 45 to 50 days; therefore, their effectiveness will be minimal by the time the leafhoppers arrive, usually in early June. Even when Furadan or Lorsban is used at planting, spray protection may still be needed before harvest, depending on the buildup of leafhoppers in the field.

Regrowth after the first cut must be monitored closely for insects, starting when the regrowth is 2 to 3 inches tall. See Tables 8-6 and 8-7 for materials and rates.

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

Leafhopper populations often vary considerably from one field to the next. For this reason, it is advisable to make leafhopper checks with an insect net in each alfalfa field. Start checking new seedings in early June, and check the regrowth of established stands when the plants are about 3 inches high. Adult potato leafhoppers are yellowish green and about .13 inch long and .03 inch wide. The nymphs are similar in appearance but lack wings. Damaging populations may be more likely when temperatures are between 70 and 90 degrees F, harvest is delayed, or alfalfa is strip-cut or cut in blocks.

Meadow spittlebug damage is most likely on legumes seeded in small-grain stubble. Spray applications are not profitable unless there are one or more spittle masses per stem by mid-May.

The adult spittlebug is .25 to .38 inch long and resembles a frog; its head is short and blunt with large eyes. Adults vary in color and marking, ranging form light grey to dark brown, with spots, strips, or bands on the wing covers. Adults walk with their front four legs and drag their back legs. The nymphal stage is found within the frothy spittle mass that they secrete. They are about .03 inch long and orange. As they develop, they become greenish yellow and then green.

Eggs are laid during August and September in small-grain stubble, alfalfa, or weeds where they overwinter. They begin to hatch during April in Pennsylvania. The nymphal stage lasts approximately 5 to 8 weeks. Adults appear in late May and early June to lay the next year's eggs.