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.
Alfalfa also known as lucerne, is an important forage crop grown around the world to provide nutrition for livestock. However alfalfa plants can fall victim to various pests that damage the crop and reduce yields. One such pest is the slater, also called the woodlouse or pill bug. Slaters are small, gray crustaceans that can infest and feed on alfalfa plants, requiring action to control their populations. In this comprehensive guide, we will look at identifying slater infestations, understanding their effects, and most importantly, effective methods to get rid of slaters on alfalfa plants.
What are Slaters?
Slaters, scientifically known as Oniscidea, are a group of small terrestrial crustaceans that are closely related to shrimp and crabs There are over 3,600 species of slaters worldwide These nocturnal creatures prefer damp, dark environments and feed on decaying plant material. During the day, they hide under rocks, boards, leaf litter, compost piles, and mulch. At night, they become active and forage for food.
A key identifying feature of slaters is their ability to roll into a tight ball when disturbed, which gives them the alternate name “pill bugs.” Slaters breathe through gills and must remain in moist environments to prevent dehydration. They are typically gray or brown in color and grow up to 1⁄2 inch in length when fully mature.
Effects of Slaters on Alfalfa Plants
While slaters mainly consume decaying plant matter, they can also feed on live plants, including alfalfa. Their chewing damage on foliage and stems weakens and stunts the growth of alfalfa plants. High populations of slaters can quickly defoliate entire sections of an alfalfa field.
Signs of slater damage on alfalfa include
- Irregular holes in leaves
- Chewed leaf margins
- Defoliation
- Wilting and stunted plants
- Presence of slaters around base of plants
Left uncontrolled, slaters may destroy alfalfa seedlings and reduce yields in older plants. Their feeding damages the plant’s ability to photosynthesize and transpire properly. Severe infestations can potentially kill alfalfa plants.
Conditions That Encourage Slaters
To get rid of slaters on your alfalfa crop, it helps to understand what environmental conditions encourage their populations to grow. Slaters thrive in the following situations:
- Excess moisture – wet soil, standing water, overwatering
- Dense ground cover – heavy mulch, leaf litter
- Mild temperatures – they dislike extreme heat or cold
- Plentiful decaying matter – compost, manure, crop debris
- Reduced sunlight – shade, cloudy weather
Any steps you can take to alter these conditions to be less favorable for slaters will help control their numbers. Improving drainage, removing ground debris, and allowing sunlight to reach the soil surface all help make the habitat less hospitable to slaters.
Methods to Get Rid of Slaters on Alfalfa Plants
If you discover slaters are damaging your alfalfa plants, prompt action is needed to protect your crop. Here are some of the most effective methods to get rid of slaters:
1. Encourage Natural Predators
Promoting the presence of slater predators like birds, frogs, lizards, and centipedes through habitat management can help reduce slater populations naturally. Create an environment around your alfalfa field that provides food, water, and shelter to attract these predators.
2. Eliminate Excess Moisture
As mentioned earlier, slaters need a moist environment to survive. Eliminate sources of excess moisture by:
- Improving drainage in the field
- Repairing leaky irrigation
- Allowing the soil surface to dry between waterings
- Thinning dense ground cover to promote evaporation
This removes the damp conditions slaters depend on.
3. Remove Ground Debris
Debris like piles of manure, dead vegetation, leaf litter, and heavy mulch provide shelter and food for slaters. Regular removal of this material deprives slaters of harborage and breeding sites.
4. Apply Diatomaceous Earth
This powdery natural substance works mechanically by causing microscopic cuts in the waxy outer layer of slaters, leading to dehydration and death. Lightly dust diatomaceous earth on and around plants. Reapply after rain or watering.
5. Use Traps
Place damp cardboard rolls or overturned citrus peels near plants at night. Slaters will be attracted to these dark, moist areas. Collect traps in the morning and dispose of captured slaters in soapy water.
6. Apply Organic Insecticidal Soap
Insecticidal soaps containing potassium salts of fatty acids can help deter and kill slaters through disruption of cell membranes. Spray infested plants, taking care to coat undersides of leaves.
7. Introduce Slater Pathogens
Nematodes or other pathogens that specifically infect slaters can be purchased and released to reduce their numbers. Always follow label directions when applying biological controls.
8. Use Protective Row Covers
Floating row covers of fabric or plastic create a temporary barrier that prevents slaters from accessing plants. Useful for protecting seedlings while plants are small.
9. Cultivate Soil
Tilling the soil can help disrupt and expose slaters to predators and harsh environmental conditions.
10. Practice Crop Rotation
Rotating alfalfa annually with a non-host crop limits build up of slaters that can occur when alfalfa is continuously grown in the same field.
Through diligent monitoring and timely application of one or more of these methods, slater infestations can be successfully controlled to prevent further damage to alfalfa plants. Always check for the presence of slaters at night when they are active. Be prepared to take action at the first signs of infestation. By creating an unfavorable environment and using integrated pest management strategies, you can protect your alfalfa crop from destruction by invasive slaters.
Preventing Future Slater Infestations
Once you have succeeded in eliminating slaters from your alfalfa planting, a little prevention can help keep them from reestablishing themselves:
- Maintain proper moisture levels in the soil
- Quickly replace damaged irrigation lines
- Remove weed species that act as alternative slater hosts
- Allow alfalfa field to dry out completely between cuttings
- Keep surroundings free of debris piles
- Mow and maintain buffer areas around field
- Rotate alfalfa annually with a non-host crop
- Scout regularly for signs of slater damage
- Be prepared to implement control measures at first sign of slaters
Stopping an infestation before it has a chance to build up is much easier than trying to control a heavy, established slater population. By making your alfalfa field an inhospitable habitat for slaters and closely monitoring your plants, you can avoid catastrophic crop losses from slater feeding damage.
Slaters can be a frustrating pest for alfalfa growers. However, this guide outlines several strategies that can be employed to successfully get rid of slaters on your alfalfa plants. The key is being vigilant in scouting for these nocturnal pests, altering environmental conditions to be less favorable, and utilizing biological, mechanical, and physical controls as needed to reduce slater populations. With a proactive integrated pest management approach, slater infestations can be contained, resulting in healthy, productive alfalfa fields.
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.
Scouting for Cutworms in Alfalfa
FAQ
What is the best killer for Slaters?
How to get rid of alfalfa weevil naturally?
What do you spray on alfalfa for bugs?
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Pest
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Active Ingredient(s)
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Trade Name(s)
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Alfalfa Weevil Larva
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alpha-cypermethrin*
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Fastac
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Alfalfa Weevil Larva
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beta-cyfluthrin*
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Alfalfa Weevil Larva
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cyfluthrin*
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Alfalfa Weevil Larva
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gamma-cyhalothrin*
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How to get rid of alfalfa worms?