Oklahoma State University Stillwater, OK
Oklahoma Alfalfa Oklahoma Cooperative Extension Service Oklahoma Agricultural Experiment Station

Alfalfa Weevil
Life Cycle and Damage. The alfalfa weevil is a snout beetle about 1/4 inch in length and light brown in color with a dark brown, mid-dorsal band; This insect completes one generation per year. The life cycle begins in late fall when adult weevils enter alfalfa fields from their summer resting sites (dense vegetation such as fence rows and along creek bottoms). Adults normally begin laying eggs inside stems of alfalfa plants during November and continue until April or May of the following year. Egg lay (oviposition) continues throughout winter and large numbers of eggs may accumulate before hatching begins during January or February. As they hatch, larvae leave the stems and crawl to growing alfalfa terminals to begin feeding. Newly hatched larvae are yellow in color and feed on developing leaf tissue within plant terminals. As they grow, larvae feed in more exposed locations on leaves and can be recognized by their green color and shiny black head capsules. Larval numbers usually increase rapidly as plant growth begins during February and March, and the potential for defoliation in early vegetative growth is great. The peak in larval numbers and greatest amount of damage occurs during March and April (Figure 2-1). Figure 2-1.  Graph of Alfalfa weevil egg and larval occurrence Yield reduction of alfalfa due to defoliation by weevil larvae is about 170 lb/acre in the first crop for each increase of 1 larva/stem in the population. An additional reduction of about 140 lb/acre occurs in the second crop due to loss of vigor in damaged alfalfa stands. With combined losses due to actual feeding damage in the first crop and residual effects later totaling more than 300 lb/acre for each increase of 1 larva/stem, it is essential that timely, effective insecticide applications be made when results of field scouting indicate that weevils are present in high numbers and potential losses due to feeding will exceed the cost of control (at the economic threshold). The time period when larval numbers exceed the economic threshold may vary greatly by year and location in Oklahoma, ranging from late February to mid April. Figure 2-2.  Graph of infestation vs. yield reduction  Sampling and Control. The scouting procedure designed for use in decision-making relative to application of chemical insecticides takes into consideration, 1) accumulated heat energy (degree-days) for weevil development, 2) alfalfa plant height, and 3) number of weevil larvae collected in a 30-stem sample (OSU Current Report No. 7177). Degree-day values for each county can be obtained from the Oklahoma Mesonet System. Specific values obtained for these three variables at the time of sampling are combined in a recommendation chart to determine if spraying is warranted. If spraying is not recommended on a particular sampling date, a time interval for resampling is given. This is the most accurate sampling procedure for decision-making, particularly when sampling after application of insecticide or following a period of freezing weather that may have killed some larvae. When a recommendation to spray has been obtained from the scouting process, it is important that label instructions be followed relative to rates of insecticides and that applications be made with favorable weather conditions; temperature above 50o F and wind velocity less than 10 mph. For both ground sprayers and aerial application, swath width should not exceed the length of the spray boom to avoid having skips in coverage. For effective control of weevil larvae, it is important that adequate spray volume is used for thorough coverage of alfalfa foliage; a minimum of 10 gal/acre with conventional ground equipment and 2-4 gal/acre for aerial application is recommended . The value of increasing spray volume for effective control of the alfalfa weevil is illustrated in Table 2-2. Table 2-2. Effects of increasing spray volume for control of alfalfa weevil with aerial application. Spray volume Days after application Gallons/acre 3 days 7 days 14 days ---------Weevil larvae/stem --------- 0.5 1.5 1.6 3.0 1.0 1.0 1.4 2.2 2.0 0.9 1.1 2.4 4.0 0.8 0.8 1.9 Unsprayed 3.0 3.8 9.9 Application of insecticides has been an essential component of control programs for the alfalfa weevil in Oklahoma since 1970. Nevertheless, populations of this pest and the potential for losses can be reduced by other means such as planting varieties with tolerance to insects and by grazing of alfalfa stands for 2-3 weeks during December and January. Grazing can reduce number of alfalfa weevil eggs in March by 50-70% (compared to nongrazed). Recent studies have also shown that grazing during March can remove nearly all eggs and larvae present, and eliminate the need for spraying. Grazing in March is however usually not recommended in hay fields due to bloat and increased weed problems (see "Integrate Grazing into Harvest of Thinning Stands" in the Weeds Management section of this chapter). Two natural enemies of the alfalfa weevil are wasps that parasitize weevil larvae and a fungal disease that kills both larvae and pupae. These beneficials are most important in killing weevils that remain in stands near the time of first cutting. The fact that weed growth is increased in areas that are defoliated gives an added incentive for maintaining an effective control program for the alfalfa weevil. Additional information relating to control of the alfalfa weevil can be found in OSU Extension Facts No. 2097 and Oklahoma Alfalfa.
Richard Berberet, Alfalfa Entomologist  Department of Entomology and Plant Pathology  Oklahoma State University Phil Mulder, Extension Entomologist  Department of Entomology and Plant Pathology  Oklahoma State University

 

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