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Alfalfa Weevils are Hatching!

Posted: January 22, 2008

Del Voight, Extension Agronomy Agent, Capital Region, Pa.

The mild winter has enabled many insects to increase in population and, in some cases, damage area crops. Already we have many insects that are damaging timothy, orchardgrass, small grains and alfalfa. This article details the alfalfa pests that are important today.

Area fields are infested with both alfalfa weevils and pea aphids of which I will detail alfalfa weevils. Alfalfa weevils have grown in population over the last few years and have accounted for huge losses in income for area producers from their feeding on the leaf portion of the plant reducing the relative feed value and protein content of the hay.

Weevils follow a lifecycle referred to as complete metamorphosis (change). This means the insect progresses through an egg, larvae and adult stage that appear to be completely different in appearance but are still the same insect.

Adults which are hard bodied dark colored insects with a distinct snout, search for alfalfa in the fall that has as many egg
laying sites as possible. Fields that are mowed close to the ground in the fall eliminate egg laying sites for these adults
(Dively U of Maryland). Those same adults awaken in the spring and begin laying more eggs in alfalfa fields. In general the fall laid eggs account for 20% of the total hatch in the spring and the spring laid eggs account for 80% of the total spring hatch. Adult females can lay more from 500 to 2000 eggs so there can be significant numbers if left unchecked by growers.

Today one can walk alfalfa fields and find larvae feeding on alfalfa that are the result of the fall laid eggs. Just last week I received reports of adults in area fields and laying more eggs which will end up as the official spring hatch. These eggs should hatch in the next seven days and begin feeding immediately.

Entomologists at Penn State and other Universities can predict the activity of the pest and potential damage through the use of heat units. This method is simply a measure of the accumulated heat from January 1. By following the heat we can predict egg hatch, larvae growth and more importantly the period which the insect is no longer a problem. Generally it takes from 14 to 21 days from egg to adult depending on heat accumulation.

This year we had first egg hatch the last week of March indicating the fall laid eggs. Last year this did not occur until the 19th of April showing just how far ahead we are this year.

Eggs, over-wintering in the stem of the alfalfa, hatch into a pale green worm with a distinct white stripe down its back. The first instar(stage) of the weevil are very small and hide in the bud portion of the plant. The best indication of their activity in the field is small shot holes that are visible around the new growth of the plant. Once the larvae get to the third instar @1/4 to1/2 inch they will do massive damage to the plant and in some cases can remove all foliage from the plant.
Observing recent heat unit patterns and the next two week forecast it appears that the third instar stage for fall laid eggs will occur from April 14-April 21 2002. This will be the time that growers will need to have already determined the need for treatment. Spring laid eggs should be active at this time as well and growers will need to time crop protection product
applications to correspond with these dates.

To scout for this pest, growers should be collecting (randomly) 30 stems from the field. Using a bucket, the scout will tap the stems along the edge of the bucket. The larvae will fall to the bottom of the bucket and can be counted by the scout. If
more than 60 larvae are counted then a treatment is warranted immediately. Another method is to observe leaves and if four out of ten leaves have shot holes then treatment is called for.

Widespread depletion of the population is of major concern. Fungal pathogens as well as parasitic wasps need some weevils around in order for them to survive. These biological weapons accounted for the massive drop in weevils populations 15 years ago. The fungal pathogen requires warm and damp weather to infect larvae and kill them.

If the conditions exist many it could well mean an application of an insecticide is not warranted. In addition parasitic wasps place eggs inside the adult weevils and the egg eats the weevil adult inside out. If growers see large amounts of either disease of these wasps then the avoidance of chemical control may be the result.

If crop protection products are needed growers have many options available to them. Applications using synthetic pyrethroids (contact insecticides) such as Warrior, Baythroid or Pounce need to come in contact with the insect to be effective. For this reason, growers need to increase application rates to a minimum of 20 gallons per acre and preferably with pressures exceeding 40 psi. Applications of organophosphates (Furadan, Dimethoate, Lorsban), which use both contact and systemic pathways to insects, require applications in warmer weather to be highly active. For this reason, growers need to allow for warm weather (60 degree F) to improve the effectiveness of these materials. Also carbamates such as Sevin are effective but also must come in contact with the pest similar to synthetic pyrethroids.

Weevils are active now and growers need to take action now to determine the need for treatment. This spring suggests a similar pattern to spring 2000 and we may see weevil activity on the regrowth of second cutting so growers must be scouting weekly to determine additional control measures.