Georgia has about 30,000 acres of alfalfa. Most is grown for hay or silage for dairy operations but about a quarter is grown as high-quality hay for horses. The No. 1 insect pest is the alfalfa weevil. It is a perennial pest requiring control in most fields almost every year. Estimated losses for the state in 2006 totaled $347, 000 with $249,000 being from yield losses and $98,000 from the cost of control measures. This translates into an average loss due to alfalfa weevil of $11.50 per acre.
Other pests are more sporadic and include the potato leafhopper, aphids and threecornered alfalfa hopper. Losses from these pests vary from year to year but usually are less than $40,000 statewide in any year. Blister beetles also a major concern to producers growing alfalfa for the horse hay market.
Alfalfa weevil larvae feed within the plant terminals, causing a “ragging” of leaves emerging from the terminal buds. This damage reduces forage yield and quality and may delay development of the first cutting. In Georgia, larvae are present in damaging numbers early in the first growth cycle, late February through mid-March, when stems may still be short. This is normally several weeks before harvest, so early cutting to control alfalfa weevil is not an option.
Larvae can be sampled using either a sweep net or a shake-bucket technique. For the shake-bucket technique, carefully pick 30 stems per field randomly, place them in a white plastic bucket and beat stems against the side of the bucket. For treatment thresholds, click here. Shake-bucket thresholds are based on stem height. If stems are very short and most larvae are very small (1/16” long), most likely eggs are still hatching and treatment should be delayed to allow most eggs to hatch before treatment.
At least one insecticide application will be needed in most established alfalfa fields every year to control alfalfa weevils. Numerous insecticides are available for alfalfa weevil control. Furadan 4F has been used in the past but several pyrethroid insecticides are currently used. Pyrethroid insecticides include lambda cyhalothrin (Warrior 1.0, Silencer, Lambda, others), gamma cyhalothrin (Proaxis 0.5), zeta-cypermethrin (Mustang MAX, Respect 0.8) and beta-cyfluthrin (Baythroid XL 1.0). The cyhalothrin products generally have a longer activity than zeta-cypermethrin, therefore the high rate range (3.2-4 fl. oz./acre) rate in recommended for zeta-cypermethrin, whereas the lower rate of 2.56-3.2 fl. oz. rate is effective for Proaxis and 1-lb/gal formulations for lambda cyhalothrin. Zeta-cypermethrin has a three-day harvest interval for hay, while the others products have a seven-day harvest interval. Steward 1.25EC also is effective against alfalfa weevil at the lowest label rate of 6.7 fl. oz. per acre. Steward is fairly selective and does not disrupt most natural enemies, but it also does not control aphids.
When selecting an insecticide for alfalfa weevil, carefully consider the product harvest interval. Some older products at higher rates, such as Furadan and Lorsban 4E and similar products, can have very long harvest intervals.
Potato leafhoppers sometimes reach damaging levels in later alfalfa growth cycles in July through September in Georgia. They feed on leaves and stems by sucking plant fluids. Their feeding causes a toxic reaction that typically causes the tips of leaves to turn yellow. This injury forms an inverted V-shape from the tip and is called ‘hopperburn.’ Severe damage will stunt plants and make the stand appear yellowed. Potato leafhopper damage is sometimes mistaken as a nutrient deficiency or disease.
Alfalfa varieties with resistance to potato leafhoppers have been developed and may be available. These varieties generally are more tolerant of leafhopper damage. However, few varieties adapted to the Deep South have potato leafhopper resistance and this resistance has not been extensively evaluated in Georgia. Generally, pyrethroid insecticides as listed for alfalfa weevil and dimethoate (various formulations and brands) are recommended for control of potato leafhopper. Steward also is labeled at 9.2 fl. oz. per acre but this rate normally is more expensive than other dimethoate or the pyrethroid insecticides.
Threecornered alfalfa hoppers are so named because adults have a large triangular shield over their back. Both nymphs and adults are green and have piercing-sucking mouthparts. They feed on alfalfa stems that may girdle the stem, causing it to wilt and die. Damage in alfalfa is cryptic and usually not recognized, so losses are difficult to gage. Alfalfa in Georgia usually is not treated for this insect but it is one pest that deserves more attention. Treatment threshold is 10% of stems being girdled and dying. The pyrethroid products listed for alfalfa weevil also are available for control of threecornered alfalfa hopper.
Aphids can occasionally be serious pests of alfalfa in Georgia. They are often especially damaging during years with cool, wet conditions. Aphids insert their mouthparts into plants and feed directly on plant juices. Their feeding usually does not cause obvious yellowing or stunting but large numbers can reduce plant vigor and growth.
Two aphid species typically attack alfalfa in Georgia. The pea aphid is light green and usually occurs on the first crop of the season. The spotted alfalfa aphid is smaller than the pea aphid and is pale yellow to white with several rows of dark spots along its back. Damage symptoms appear as a yellowing of the foliage between the leaf veins. Some alfalfa varieties have resistance to these aphids. Alfalfa is fairly tolerant of aphid injury, and treatment thresholds are listed in the table. If alfalfa is within one week of cutting, harvest early and treat stubble as needed. Natural enemies such as lady beetle larvae and adults and various parasites usually keep aphid populations in check. Lady beetles are not active during the winter, so large aphid populations may build up in late winter on non-dormant alfalfa before lady beetles become active in spring. If spotted alfalfa aphids appear in seedling stands, treat when they can be easily found in the field, because this aphid is capable of severely stunting or destroying alfalfa seedlings. Dimethoate and cyhalothrin products are recommended for aphid control. Zeta-cypermethrin and Baythroid are not effective against alfalfa aphids.
Several other insects, such as leaf-feeding caterpillars, cutworms and grasshoppers, defoliated alfalfa and occasionally reach damaging levels. Control measures are recommended when defoliation exceeds 10%.
Blister beetles feed on foliage but rarely cause significant defoliation. Instead, they contain a very toxic chemical in their body that contaminates hay. This compound, cantharidin, can be very toxic to livestock – especially horses. The stripped blister beetle is the species usually associated with alfalfa hay poisonings. For adult horses, several dozen blister beetles are needed to cause acute poisoning. Generally, the first cutting is at lowest risk of infestation, because beetle populations peak in late summer. But the risk of contamination is present for every cutting. Blister beetles can aggregate in very large number often along the field margin. Modified harvesting methods, such as not driving over previously cut hay, can reduce the risk of blister beetles contamination. But with current technology it is not possible to completely eliminate the risk of contamination by blister beetles.
For more detailed information on blister beetle management in alfalfa hay, see UGA Extension Circular 917 (http://pubs.caes.uga.edu/caespubs/pubs/PDF/C917.pdf).
Finally, proper crop management can insure high alfalfa yields and quality. Following recommended agronomic practices (fertilization liming, weed control, seeding, etc.) to achieve and maintain vigorous, healthy stands allows alfalfa to tolerate and/or outgrow insect damage more efficiently than plants not receiving proper care.Recommendations listed in this article apply to Georgia, and may not applicable to other states. Alfalfa producers should contact their local extension service for specific recommendations for their area.