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Alfalfa Weevil

CROPS IMPACTED: Alfalfa

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Family: Curculionidea
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About the Alfalfa Weevil

Reproduction and Life Cycle

The alfalfa weevil overwinters as an adult in leaf debris and soil where alfalfa is grown. They will surface early in the spring, usually once temperatures are 60 degrees Fahrenheit or higher and will immediately begin feeding off of the alfalfa leaves. The weevil will form round holes in the leaves they eat. This will continue for several weeks. Once the female is fertilized, she will chew into the alfalfa stem and place up to 40 eggs. The eggs can take 7-14 days to hatch. The larvae will then stay in the stem for several days, and eventually move to new leaf buds. Later in development, the larvae will start to feed on leaves until they are ragged and almost skeletonized.

Alfalfa Weevil Identification and Habitat

Identification

When alfalfa weevils reach full maturity, they become brown with a dark stripe running down its back and are about 5mm long. With their hook-like front, they are classified as a snout beetle. Alfalfa weevil eggs are very small and take on a bright yellow colour. When they reach maturity, they will darken slightly. Newly hatched larvae are a yellow-green and have black heads with a white stripe on its back. When the larvae mature, they grow to be roughly 8mm long and eventually change to be a dark green.

Habitat

The alfalfa weevil is common throughout the United States and eastern Canada; recently an increasing problem in Saskatchewan. They are currently the most concerning pest for alfalfa plants. They have the ability to destroy all the leaves on a plant, which will downgrade its quality and yield. The main damage tends to occur during the first cut of the season, leading to a negative outcome for the second cut as well. Alfalfa weevils may also interest themselves with plants such as white and red clover, white sweet clover, and yellow sweet clover. The larvae will cause the greatest amount of damage to the plant as they have the ability to eat the entire leaf. However, adults are still harmful, eating away at leaves, main stems, and side shoots. When an entire field is infested it will take on a grayish colour, almost like a mist throughout the field.

Alfalfa Weevil Management and Control Methods

Management

Having satisfactory control over the alfalfa weevil is dependent on using an integrated management system that includes the use of cultural and chemical methods. Only use insecticides when crop benefits outweigh the cost of treatment.

Cultural Control

Planning an early harvest can greatly reduce alfalfa weevil population. This is due to the fact that at this time in the season, the weevil population is so high that when you cut the alfalfa, you eliminate a large portion of the beetles shelter and food and expose them to the hot sun. There are also other species that can help control the weevil. A fungus called Zoophthora phytonomi can kill a large portion of the larvae population. This fungus can be spread with a windrower, which is also why having an early harvest can be extremely effective. Parasitic wasps, lady bugs, damsel bugs, and lacewings can also help control the alfalfa weevil.

Chemical Control

When alfalfa weevil populations are extreme and beyond the economic threshold, chemical control will be necessary for effective control. Some chemicals that have proven to have an effect on the alfalfa weevil is Matador/Silencer, Dicis 5EC, Malathion 500, Malathoin 85E (only effective on larvae), Imidan, and Lagon. Be sure to carefully read insecticide labels for cautions and proper application.

Latin / Alternative Alfalfa Weevil Names

  • • Hypera Postica

Sources

http://www.agriculture.gov.sk.ca/Alfalfa-Weevil

http://www.ext.colostate.edu/pubs/insect/05500.html

http://ento.psu.edu/extension/factsheets/alfalfa-weevil