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How to scout for alfalfa weevil

In our area, alfalfa acres are far fewer than those planted to corn, soybean and wheat. Alfalfa is grown to provide a highly nutritious hay for livestock such as cattle, horses and sheep.
Alfalfa plants are perennial and may live four to five years. Established alfalfa stands will break winter dormancy and start new growth each spring. These stands will be the first crop to see potential insect pests, such as the alfalfa weevil.
The alfalfa weevil is a small, brown, snout-nosed beetle approximately 3/16 inch in length with a wide dark stripe down its back. The larva is green with a black head and a white stripe down its back.
Larvae pass through several developing stages. Both the adult and larvae feed on alfalfa foliage. Larvae cause most of the damage; adult feeding is generally not significant. Severe larval feeding can greatly reduce yields and affect forage quality.
Generally, alfalfa weevils only affect the first hay cutting. During periods of heavy weevil activity, early growth of the second cutting may be impacted.
Young larvae feed primarily on growing tips. Large numbers of more mature larvae may extensively defoliate plants.
The life cycle of the alfalfa weevil begins with the adult, which is the predominant overwintering stage. In the spring, when temperatures begin to exceed 48 degrees F, the adults become active and will deposit nine to 10 eggs in clusters on alfalfa stems throughout a field.
As heat units above 48 degrees accumulate, the eggs hatch and larval development proceeds. Larvae reach a size that may cause serious feeding damage between 325 and 575 growing degrees. Currently, about 360 growing degrees have accumulated in the Hancock County area.
The following website shows how to calculate growing degree days: https://www.oardc.ohio-state.edu/gdd/glossary.asp
Larvae will eventually mature and spin a fibrous net cocoon and transform into pupae, from which the adult stage emerges. The pupae become the predominant stage as accumulated growing degrees reach 800. In Ohio, the life cycle of the alfalfa weevil normally is limited to a single generation per year.
Over the past few decades, populations of alfalfa weevil have seldom reached levels that require insecticide treatment because of an introduced biological control, which consists of a complex of three parasitic wasps and a fungal pathogen.
Mature larvae that are biologically controlled by the fungal pathogen, Erynia sp., will be brown in color and found attached to the foliage. These biological control organisms tend to regulate the alfalfa weevil populations to a point that significant feeding damage seldom occurs.
However, in recent years, the biological control has not been as effective and farmers will scout their fields to ensure that the amount of feeding damage does not warrant an insecticide treatment.
To scout for alfalfa weevil, larvae number and alfalfa stem height will need to be determined. Farmers estimate feeding activity by carefully cutting 10 stems at ground level from three to four locations in a field.
At each location, the 10 collected stems are vigorously shaken in a bucket to capture feeding larvae. Farmers also measure the length of at least five stems at each location to estimate plant height.
The number of larvae collected from each location as well as the length of the alfalfa stems are averaged to give a value for the field. An insecticide should be applied if the average larval number is one or more per stem and the stems average less than 12 inches long.
Alfalfa stem length between 12 and 16 inches requires an insecticide if the crop is under an environmental stress and larval numbers average two to four per stem.
Early alfalfa harvest is recommended, rather than an insecticide treatment, if the alfalfa is taller than 16 inches and the average alfalfa weevil count is more than four larvae per stem. After harvesting alfalfa early, fields should be scouted for larvae on the plant regrowth.
Almost all insecticides are labeled for alfalfa weevil. Farmers need to check the pre-harvest label restriction since the interval ranges from 0 to 21 days depending on the insecticide product. Fortunately, insecticides are often not needed since parasitic wasps and/or a fungus control the weevil in most years.
For additional information, Ohio State University has a fact sheet on the alfalfa weevil at the following site: http://ohioline.osu.edu/factsheet/ENT-32.
Lentz is extension educator for agriculture and natural resources for The Ohio State University Extension Service in Hancock County. He can be reached at 419-422-3851 or via email at lentz.38@osu.edu.
Lentz can be heard with Vaun Wickerham on weekdays at 6:35 a.m. on WFIN, at 5:43 a.m. on WKXA-FM, and at 5:28 a.m. at 106.3 The Fox.



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