Alfalfa Weevil

Amy Timmerman - Extension Educator Author

05/11/2022 Added

3 Plays

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Alfalfa weevil larvae have been reported feeding in alfalfa in southeast Nebraska. As temperatures warm up, appearances in northern Nebraska in the next two weeks will occur.

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[00:00:00.315]This is Amy Timmerman with this week’s
[00:00:02.125]Extension Update
[00:00:04.675]Alfalfa weevil larvae have been reported
[00:00:06.829]feeding in alfalfa in southeast Nebraska.
[00:00:09.330]As temperatures warm up,
[00:00:11.223]appearances in northern Nebraska
[00:00:13.140]in the next two weeks will occur.
[00:00:15.003]The larvae of alfalfa weevils feed
[00:00:17.053]on first cutting alfalfa as larvae,
[00:00:19.261]and adults (and sometimes larvae)
[00:00:21.509]feed on the regrowth
[00:00:22.789]after the first cutting.
[00:00:24.635]Even with the pressure
[00:00:26.155]of planting row crops,
[00:00:27.665]it is essential that producers
[00:00:29.295]growing high quality alfalfa hay
[00:00:30.985]make time to monitor fields
[00:00:32.695]for weevils now and over
[00:00:33.935]the next several weeks.
[00:00:35.578]In the northern tier of counties,
[00:00:37.610]there may be two flushes of
[00:00:39.170]weevil larvae this spring,
[00:00:40.820]leading to regrowth damage
[00:00:42.560]after the first cutting.
[00:00:44.103]Observations indicate the cause may be due
[00:00:47.013]to significant survival of both
[00:00:48.768]adult and larval weevils.
[00:00:51.360]Most alfalfa weevils overwinter as adults,
[00:00:54.676]become active as temperatures
[00:00:56.466]increase and lay eggs.
[00:00:58.166]As a reminder we are located in the area
[00:01:00.742]of the state where there may be two
[00:01:02.912]flushes of weevil larva in the spring.
[00:01:05.442]In the last few years, some areas of the
[00:01:07.482]state have received damage to regrowth
[00:01:09.432]after the first cutting due to a combination
[00:01:12.262]of late larval feeding and adult feeding.
[00:01:14.752]This is something to be aware of
[00:01:16.546]after the first cutting.
[00:01:18.724]While alfalfa weevil damage
[00:01:20.496]has been spotty in much of
[00:01:22.138]Nebraska over the past few years,
[00:01:24.149]the potential for damage always exists.
[00:01:26.843]Alfalfa weevil damage consists of small
[00:01:28.802]holes and interveinal feeding on the
[00:01:30.939]newest leaflets near the stem tips.
[00:01:33.998]The larvae are small
[00:01:35.772](1/16 to 3/8 inch long) and
[00:01:39.353]pale yellowish green, becoming a
[00:01:41.436]darker green when larger.
[00:01:43.853]These legless worms have black heads
[00:01:46.225]and a white stripe the length of the back.
[00:01:48.721]The alfalfa larvae spend nearly
[00:01:50.634]all their time on the plant.
[00:01:52.745]They curl into a C-shape when disturbed.
[00:01:55.866]Once the alfalfa is high enough
[00:01:57.873]to use a sweep net,
[00:01:59.321]take a sample to establish
[00:02:01.020]whether weevils are present.
[00:02:03.259]If they are, randomly select
[00:02:05.082]at least five sampling sites
[00:02:06.818]from across the entire field.
[00:02:08.285]At each site, gently pick or cut
[00:02:10.756]at least 10 alfalfa stems at ground level.
[00:02:13.938]Shake the larvae off the stems by
[00:02:16.019]beating the stems into a deep-sided bucket.
[00:02:18.810]Count the larvae and determine the
[00:02:20.465]average number of larvae per stem.
[00:02:22.470]Make sure to check for small larvae
[00:02:24.556]that may be enclosed in new,
[00:02:26.469]folded leaflets at the tips of the stems.
[00:02:28.922]Measure stem lengths and determine
[00:02:30.967]the average stem height.
[00:02:32.672]To determine is action needs to occur
[00:02:35.002]refer to the economic thresholds that have
[00:02:37.987]been developed to aid decision making
[00:02:40.564]on alfalfa weevil control.
[00:02:42.152]These thresholds were derived by
[00:02:44.342]North Dakota State University entomologists
[00:02:47.212]from a two-year study conducted at the
[00:02:49.417]University of Nebraska-Lincoln
[00:02:51.025]Eastern Nebraska Research,
[00:02:53.773]and Education Center
[00:02:55.959]near Mead in 1990 and 1991.
[00:02:59.359]These guidelines can fluctuate depending
[00:03:01.495]on growing conditions and variety.
[00:03:03.892]The economic thresholds for alfalfa weevil
[00:03:06.387]can be found at cropwatch.unl.edu.
[00:03:10.232]Because alfalfa weevil natural enemies
[00:03:12.642](e.g., lady beetles and parasitoid wasps)
[00:03:16.352]have the potential to keep weevils
[00:03:18.589]from reaching economic injury levels,
[00:03:20.943]use insecticides only when necessary.
[00:03:23.887]Many insecticides are registered to control
[00:03:26.578]alfalfa weevil larvae.
[00:03:28.449]See the most recent edition of the
[00:03:30.499]Guide for Weed Management in Nebraska
[00:03:32.797]with Insecticide and Fungicide
[00:03:34.744]Information (EC130) for rates and
[00:03:36.752]restrictions of commonly used insecticides
[00:03:38.831]for alfalfa weevil larval control.
[00:03:41.360]They differ in their modes of action
[00:03:43.170]and pre-harvest intervals.
[00:03:44.841]If you have questions regarding
[00:03:46.461]insecticide selection and economic thresholds
[00:03:49.461]please reach out to your local extension office.
[00:03:52.221]This has been Amy Timmerman
[00:03:54.040]with Nebraska Extension.

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Alfalfa Weevil

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