Alfalfa Forage Insect Control
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.”
Aphids
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.
Alfalfa Weevils
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.”
Foliage Feeding Caterpillars
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
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.”
Miscellaneous Pests
Potato Leafhoppers
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.
Grasshoppers
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.
Alfalfa Forage Insect Control Suggestions
To protect insect pollinators, do not spray during full bloom. If necessary to control insects for maximum seed production, use insecticides least damaging to pollinator
Alfalfa Forage Insect Control Suggestions Table
When insecticides are applied to alfalfa that will be grazed or mowed for hay, certain precautions for use of materials must be taken. The following waiting periods from application to grazing or cutting have been established:
2Alfalfa Seed Chalcid — Control with insecticides is usually unsatisfactory due to persistence of attack and restrictions against using chemicals with long residual effects. To avoid the necessity of spraying for this insect and thereby harming beneficials; alfalfa seed should be produced as early in the season as possible.
All B.t. products — 0 days to harvest, 12 hour re-entry interval.
Baythroidr — One application per cutting only, and up to three applications per season. Avoid application of this product in cotton producing areas from mid-May to late July. Do not apply by ground within 25 feet, or by air within 150 feet of any body of water. ncrease the buffer zone to 450 feet when ultra-low volume (ULV) application is made. 7 days to harvest.
Dimethoate — 10 days to harvest.
Lannate LVr — Do not apply within 7 days of cutting or allowing livestock to graze.
Malathion 5E — 0 days to harvest for up to 1.5 lbs, 5 days for more than 1.5 lbs.
Mustang MAXr — 3 days to for cutting or grazing, 7 days for harvesting seed.
Pouncer (Permethrin) — Do not apply more than 0.2 lb active per cutting. Do not apply permethrin products in cotton producing areas from mid-May to late July. Required waiting period before harvest: 0.1 lb active or less - 0 days, above 0.1 lb active - 14 days.
Proaxisr — Do not apply more than 0.24 pints per acre per cutting. Do not apply more than 0.96 pints per acre per season. Avoid application when bees are actively foraging. This chemical is Gamma-cyhalothrin; if it is used in the same season as lambda-cyhalothrin (Warriorr) then read the label carefully for use rate limitations. One day for harvest of forage and 7 days to harvest for hay.
Sevin — 7 days to harvest.
Silencerr – Do not apply more than 0.24 pts per acre per cutting or more than 0.96 qts. per acre per season. Do not apply with 1 day of harvest for forage or within 7 days of harvest for hay.
Steward — Do not apply more than 45 fl oz/A of STEWARD EC insecticide or 0.44lb ai/A of indoxacarb-containing roducts per calendar year. Apply no more than 11.3 fl oz/A of STEWARD EX insecticide or 0.11 ai/A of indoxacarb-containing products per cutting. Seven-day (7) PHI. When STEWARD EC insecticide is used on alfalfa grown for seed, the seed may not be used for sprouts or livestock feed. All seed from treated crtop must be tagged, "Not for Human or Animal Use" at the processing plant.
Vantacor — Do not make more than 4 applications per acre per calendar year. Make on application per cutting. Do not apply more than 5.1 fl oz VANTICOR insect control or 0.2 lb a.i. of chlorantraniliprole containing products per acre per calendar year.
Warriorr — Avoid application around bee shelters or when bees may be actively foraging. Do not apply more than 0.03 lb a.i. per acre per cutting or more than 0.12 lb a.i. per acre per season. One day for harvest of forage and 7 days to harvest for hay.
r Restricted use pesticides.