Insects: Alfalfa - established stands
Part 2, Section 6: Forages Pest Management
Forages Pest Management
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 can be avoided if a spray is applied at the proper time.
A good pest management program requires proper identification of the pest species causing crop injury, 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 local extension office. This publication will aid you in deciding when it is profitable to control potato leafhopper and alfalfa weevil. Pest identification requires knowing when and what to look for in the field. Figure 2.6-1 shows the key periods of forage insect occurrence in Pennsylvania.
Alfalfa blotch leafminer can be found in practically all alfalfa fields in the state; however, damage always looks worse than it actually is. The second cutting usually is most severely infested. Control may be justifiable if 30% or more of the leaflets have a mine present.
The adult fly is about 0.13 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 ovipositor 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 typically are comma-shaped. Blotches and punctures can cause deterioration of foliage quality, loss of photo-synthetic area, and defoliation.
Alfalfa weevil is a problem primarily 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. See A Pest Management Program for Alfalfa in Pennsylvania for information on the alfalfa weevil life cycle, identification, sampling techniques, and economic thresholds.
Black cutworms can cause extensive damage to new seedings in late May and early June. See Part 2, Section 2, Corn, for information on cutworm biology.
Pea aphid control sometimes is needed, but natural controls usually suffice 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 0.19 inch long. They can be winged or wingless. Like other aphids, the pea aphid damages the plant by removing sap with its sucking mouthparts 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 around 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 pests of alfalfa in the state. Many poor stands and low yields can be attributed to these pests. New spring seedings are especially vulnerable to attack by leafhoppers. Regrowth of second and third cuttings of established stands also is damaged frequently.
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. The Penn State Cooperative Extension publication A Pest Management Program for Alfalfa in Pennsylvania discusses economic thresholds. For glandular-haired potato leafhopper resistant alfalfa, studies indicate that the thresholds can be tripled to a 3× level compared to the regulary used thresholds.
Adult potato leafhoppers are yellowish green and about 0.13 inch long and 0.03 inch wide. The nymphs are similar in appearance but lack wings. Damaging populations may be more likely when temperatures are between 70 and 90°F, when harvest is delayed, or when 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 0.25 to 0.38 inch long and resembles a frog; its head is short and blunt with large eyes. Adults vary in color and marking, ranging from light gray 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 0.03 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.
Slugs can damage new seedings of alfalfa, particular fields seeded no-till in the fall. Spring seedings are less susceptible to slug injury.