Most Common Alfalfa Plant Pests: Identification and Treatment Guide

In Oklahoma, insect pests are a perennial problem that can cause reduced alfalfa productivity. These pests occur at various times of the growing season and reduce forage production in many ways.

Yellowing (chlorosis) and subsequent death of leaves (necrosis), along with stunting of plant growth, are caused by fluid feeding insects such as aphids and leafhoppers. The major aphid species include the pea, spotted, and blue alfalfa aphids. Another fluid feeder in alfalfa is the potato leafhopper.

During the spring and summer months, defoliation is a common form of damage in alfalfa. It is caused by the alfalfa weevil and several larvae. Extensive defoliation may cause significant forage yield reductions and can also reduce stand longevity if damage is severe and occurs repeatedly.

A third form of damage by insects is on new stands. During early spring and fall, larval stages of the army cutworm and fall armyworm may cause defoliation to the extent of removing all plant growth above the soil surface, and feeding by large populations may result in stand destruction. In addition, feeding by aphids during this same period can devastate young alfalfa stands, even when populations are low (<10 aphids/stem).

Still another type of damage caused by insect pests is destruction of new growth after cutting. This damage usually occurs after first cutting and is most often caused by variegated cutworms. Alfalfa weevil adults may also create a similar problem. This damage may lead to reduced growth and vigor while contributing to stand thinning. A thorough discussion of pest biology, damage, scouting, and economic threshold levels is provided in Oklahoma State University Circular E-826, “Alfalfa Production Guide for the Southern Great Plains.”

Pea and blue alfalfa aphids generally infest alfalfa fields beginning in March and may continue feeding through April and May. Spotted alfalfa aphids are usually associated with mild, dry weather conditions and may be active throughout the year. Historically, serious infestations have occurred in the fall, winter and early spring. Pea aphids are the largest of the species that occur in alfalfa. They are light green in color and possess a dark band at the base of each antennal segment. Blue alfalfa aphids are similar in appearance; however, they are usually smaller and bluish-green in color. In addition, the antennae have no bands, but gradually darken in color toward the tips. Spotted alfalfa aphids are small and yellowish-green with several rows of small, black dots along the dorsal surface. A magnifying glass or hand lens is essential in identifying these species.

The most damaging species are the blue and spotted alfalfa aphids. Both aphids cause obvious toxic effects (chlorosis and wilt) in plants while feeding. During the spring, plants can be severely stunted and are often killed when large numbers of aphids are present for several weeks.

The alfalfa weevil is the most damaging pest of alfalfa in Oklahoma. It overwinters as both eggs and adults. During mild winters in Oklahoma, larvae may appear late February or early March. However, activity generally begins in early spring with evidence of feeding on growing plant tips. Young larvae are yellowish in color, but as they mature, they turn green with black heads and possess a white stripe down the center of the back. For additional information on the development and management of the alfalfa weevil, refer to OSU Fact Sheet PSS-2091, “Alfalfa Weevil and Its Management in Oklahoma.”

In Oklahoma alfalfa, several species of foliage feeding caterpillars may be found throughout the summer. The species present include webworms, alfalfa caterpillar, green cloverworm, corn earworm, armyworm and yellow-striped armyworm. These defoliators are rarely a significant problem in established alfalfa, although seedling stands can be heavily damaged by their feeding.

Additional caterpillars that often occur at times of the year when alfalfa is particularly susceptible to defoliation include the fall armyworm, army cutworm and variegated cutworm. Fall armyworms may destroy newly planted alfalfa in the fall while army cutworms can destroy new stands in early spring. The variegated cutworm may do serious damage immediately following first harvest. Variegated cutworms that have hatched during growth of the first crop mature during first harvest and may destroy or delay regrowth of the second crop.

During mid and late summer, webworms may cause serious defoliation in Oklahoma alfalfa. They spin webbing over leaves and buds of alfalfa terminals and can cause losses in both seed and forage production. Insecticide applications for defoliators are most effective when larvae are small and/ or webbing (webworms only) is minimal.

For a detailed description of the larvae found in Oklahoma alfalfa, consult OSU Extension Fact Sheet EPP-7159, “Field Key to Larvae in Alfalfa.”

Blister beetles are slender, soft-bodied, black, gray or striped beetles from one-half to three-quarters of an inch long. Adults feed on foliage; however, larvae of species found in alfalfa are considered beneficial because they feed on grasshopper eggs. Blister beetles are gregarious, often congregating in large swarms within alfalfa fields. Swarms move frequently and are typically comprised of one or two species of small striped beetles. Blister beetles are important in alfalfa production, not so much for their foliage feeding, but because they produce cantharidin, a blister beetle defense, that is toxic to horses consuming hay that contains bodies of these insects that were killed in the harvesting process. Cantharidin is secreted from leg and body joints and is toxic even in dried bodies of dead beetles. Suggestions for management of blister beetles in alfalfa can be found in OSU Extension Fact Sheet PSS-2072, “Blister Beetles and Alfalfa.”

This small, wedge-shaped insect is light green and about one-eighth inch long. Feeding by this insect causes yellowing and necrosis of leaves that begins at leaf tips and is called hopperburn. Losses from this insect may occur during the summer months on new growth after alfalfa is harvested. Damage potential by this pest in Oklahoma is not well-known, but it is often found in numbers that are known to cause yield reductions in Midwestern states. In the Midwest, numbers as low as 0.5 leafhopper per sweep in 7-inch or shorter alfalfa are considered to exceed the economic threshold. Caution should be used in interpreting these thresholds for Oklahoma, since lower yields of summer cuttings in alfalfa may not justify money spent on control of potato leafhoppers.

During mid and late summer, several species of grass hoppers may migrate from field margins, fence rows or pastures into alfalfa. Border areas and seedling stands are particularly susceptible when field margin vegetation matures and dries. In addition, grasshoppers infesting alfalfa being grown for seed may feed on blossoms and seed pods, resulting in serious yield reductions.

Insecticide Usages on Alfalfa

The rules and regulations governing the chemicals used on alfalfa are quite rigid. This is especially true of alfalfa going into interstate commerce or alfalfa to be fed to lactating dairy animals or animals being finished for slaughter. The chemicals listed in this publication are all approved to be used at the concentration and with the limitations described. Any use of the chemical at different concentrations or a disregard for the limitations should be avoided.

Before using any chemical on alfalfa, check the toxicity, rates of application and limitations. The producer should also be aware that alfalfa is sometimes contaminated by chemical drift of pesticides being applied to adjacent fields. Remember that all pesticides should be handled with care.

Crop Hour: Alfalfa Insect Pests, Identifying Alfalfa Diseases and Forages and Soil Fertility

FAQ

What is the pest in alfalfa?

What do you spray on alfalfa for bugs?

What does alfalfa weevil damage look like?

What are the little worms in alfalfa?

Are insects bad for alfalfa?

Most of these insects are harmless or beneficial; lady beetles are an example. Pest species generally are found in numbers too few to reduce alfalfa yield. Unfortunately, in some fields and in some years, insect pest populations reach levels that could cause economic yield loss.

What are the common foliar diseases of alfalfa?

Many of the common foliar diseases of alfalfa are favored by the high moisture conditions this season in Indiana. These include common leaf spot (figures 1 and 2), Stemphylium leaf spot (figure 3), Leptosphaerulina leaf spot (or Lepto leaf spot) (figure 4), and downy mildew (figure 5).

Are there leaf spots in alfalfa?

We have received a number of calls about leaf spots occurring in alfalfa. Many of the common foliar diseases of alfalfa are favored by the high moisture conditions this season in Indiana.

Do pests affect alfalfa yield?

Pest species generally are found in numbers too few to reduce alfalfa yield. Unfortunately, in some fields and in some years, insect pest populations reach levels that could cause economic yield loss. Because of these occasional bad actors, scout alfalfa from green up through the last cutting.