Extension, College of Agriculture & Life Sciences, The University of
Tim C. Knowles, Area Extension Agent, La Paz and Mohave Counties
Description and Biology of the
Adult Egyptian alfalfa weevils (Hypera brunneipennis) are light brown
with dark brown and grey markings down their backs and are about 0.2
inches long. Adult weevils emerge from a summer resting state called
aestivation in late fall or winter and begin to migrate to alfalfa fields
to feed, mate and lay eggs. The female chews a hole in the stem then
inserts her eggs into the center of the stem. Smooth, shiny, yellow
eggs are laid in living or dead stems three to six inches above the
soil surface or in plant debris on the ground. Eggs hatch in five to
ten days. The legless larvae are initially tiny, yellowish green in
color and reach a length of 0.25 inch when fully grown. Mature larvae
are pale or light green with a thin white stripe down the center of
their back and have a dark brown to black head. Larval development takes
about three weeks and normally is completed at about the time of the
first hay cutting. After the larvae have ceased feeding, they usually
drop to the ground to spin a round closely woven white cocoon for pupation.
Pupation lasts from ten days to several weeks, then emerging adult weevils
feed for two to four weeks before they enter aestivation in weedy areas
near alfalfa fields. The adults remain inactive at these sights until
late fall or winter when temperatures rise above 42o F.
Egyptian alfalfa weevil feeds on alfalfa plant terminals on the first
growth after winter. Adult feeding seldom causes significant damage
to alfalfa but results in characteristic linear scars on stems. Weevil
larvae damage alfalfa by feeding on terminal buds and leaves first and
gradually moving down the plant. First and second instar larvae feed
on young developing leaves. Third and fourth instar larvae cause the
most damage by feeding on interveinal alfalfa leaf tissue, skeletonizing,
bronzing, and subsequently defoliating plants. Defoliation reduces alfalfa
tonnage and quality, reduces the photosynthetic capacity of the plant,
and the damage to regrowth buds can stunt growth and kill alfalfa stems.
Weevil damage is often most severe under windrows lying in the field
for several days because harvesting moves weevil larvae into a concentrated
area under cut hay where they feed on new growth. Typically, the alfalfa
weevil can cause damage to the first and occasionally to second and
third hay cuttings.
Biological and Cultural Controls
Larvae of beneficial insects including lacewings and lady beetles attack
some alfalfa weevil larvae. Two parasitic wasps have been successfully
introduced into areas throughout the range of the alfalfa weevil. Bathyplectes
curculionis parasitizes larvae of the alfalfa weevil. This parasitic
wasp has spread over much of the alfalfa weevils range since its introduction,
however, it is generally unable to suppress the weevil population below
economic thresholds. The distinctive brown, egg-shaped pupa of this
wasp has a broad white stripe around its middle and is found within
the white cocoon of the parasitized weevil. Peak activity of this parasitic
wasp is during winter or when alfalfa weevil larval populations peak.
Microctonus aethiopoides is another introduced parasitic wasp that attacks
adult female weevils, drastically reducing fall egg- laying. However
this parasitic wasp has not established itself in desert alfalfa growing
Cultural controls of the alfalfa weevil consist of practices which
produce dense vigorous alfalfa growth to reduce losses from alfalfa
weevil damage and early harvest. No varieties are resistant to the alfalfa
weevil; however, well-adapted varieties that produce rapid spring growth
and recovery can tolerate some weevil damage. Serious alfalfa weevil
damage can sometimes be prevented by cutting the crop as soon as most
of the plants are in the bud stage or by sheeping off alfalfa during
fall and winter. As alfalfa reaches the bud stage in late winter, harvesting
is normally more profitable than treating with insecticide. Sanitation
and weed control in fields and right of ways adjacent to alfalfa fields
can help reduce oversummering/aestivating adult weevil populations.
Monitoring and When to Treat
Although weevil larvae can first appear in January, peak populations
normally occur sometime between February 22 and March 23 in western
Arizona deserts. This period of peak larval activity correlates close
with the accumulation of from 445 to 495 heat units (HU) since January
1 (86/45o thresgold) and/or 800 HUs following a 42o F event. Check fields
with a sweep net every two to four days after weevil larvae begin to
appear. Divide each field into four or more quadrants and take five
sweeps of each section. Treat the field when an average sweep count
of 20 larvae per 180o or 5 to 10 larvae per 90o sweep are found, especially
when aphids are also present in high numbers. Instead of using an insecticide,
many alfalfa growers choose to harvest hay early, before larval populations
reach damaging levels, which also helps protect beneficial insect populations.
A good winter weed control program can tend to increase the number of
weevils collected in a sweep net sample, yet these fields may not show
evidence of alfalfa damage, nor require chemical control.
If sweep net counts are 10 to 15 larvae per 180o sweep (5-8 per 80o
sweep) just before cutting, then applying malathion under the windrows
may be considered. This treatment reduces feeding damage to new alfalfa
regrowth and preserves plant vigor, however use malathion only.
Carbofuran, phosmet, or methoxychlor cannot be used. At harvest under
the windrow insecticide applications require mounting a spray unit on
the swather. Another option is to apply an alfalfa stubble treatment
with permethrin plus carbofuran immediately after removing bales from
Sampling with a sweep net does not provide reliable alfalfa weevil population
estimates on young alfalfa plants or stubble following cutting. Another
sampling technique developed by the University of Nebraska (Danielson,
et. al., 1996), the stem count method, takes into consideration the
price and maturity of alfalfa to help make treatment decisions for alfalfa
weevil control. An additional advantage of this method is that it can
also be used to monitor alfalfa aphids. Check alfalfa plants in each
of four quadrants. In each quadrant, carefully cut 30 to 40 alfalfa
stems off at ground level with a sharp knife. Shake the stems or rap
them sharply onto a clean white surface such as a net, paper, or into
a plastic bucket. This will dislodge most of the alfalfa weevil larvae
that can easily be counted. While counting larvae, note the presence
or absence of lady beetle larvae, lacewing larvae, and parasitized alfalfa
weevil pupae. Further inspect each stem by prying open small new leaves
and searching the spaces between them. Record the total number of weevil
larvae counted per stem and the length of each stem, then calculate
the average number of alfalfa weevils per stem and stem length (plant
height). The following charts show when alfalfa should or should not
be treated with an insecticide to control weevils based on plant height,
weevil numbers, and crop value:
Alfalfa Weevil Stem Count Method
Currently Registered Insecticides for Alfalfa Weevil
(7 days for OilULV)
|Do not apply to wet foliage or two days
prior to irrigation to avoid injury
|½ pint = 7 days
1 pint = 14 days
2 pints - 28 days
|One application per season.
Move bees for seven days.
(Lock- On, Lorsban)
|½ pint = 7 days
1 pint = 14 days
2 pints = 21 days
|One application per cutting.
Four applications per season.
(Baythroi d 2)
||One application per cutting.
11.2 fluidoz/acre/season limit.
||Move bees for two to three days
0.96fluid pt/acre/season limit
|0 days (EC)
|Do not apply when bees are working
||Ten applications per crop
||Apply at 7 to 14 day intervals.
||Do not apply during bloom.
Hazardous to bees.
|<0.1 lb. a.i. = 0 days
>0.1 lb. a.i. = 14days
|0.2 lb. a.i./acre/ cutting limit.
Toxic to bees and fish (see label).
||One application per cutting.
Do not apply at bloom
Danielson, S., Tom Hunt, and Keith Jarvi. 1996. NebGuide: Managing
the alfalfa weevil. Guide G94-1208-A. University of Nebraska, Lincoln,NE.
IPM Manual Group. 1985. Integrated Pest Management for Alfalfa Hay.
UC Davis Press. Meister Publishing. 1997. Insect Control Guide. Willoughby,
Rethwisch, M.D. 1991. Weevils, aphids active in area alfalfa. Agricultural
Column. University of AZ Cooperative Extension, Yuma County
Summers, C.G., K.S. Hagen, and V.M. Stern. 1995. UC IPM Pest Management
Guidelines, Alfalfa pest management guidelines. University of CA, Davis.
The information given herein is for educational purposes only. Reference
to commercial products or trade names is made with the understanding
that no discrimination is intended and no endorsement by the University
of Arizona Cooperative Extension is implied.
These suggestions are not intended to take the place of product labels.
The user must accept responsibility to obtain, read, understand, and
follow all product label instructions. Label violations can lead to
civil and criminal penalties, unmarketability of crops, and could contribute
to cancellation of product labeling.
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by the University of Arizona.
Document located http://ag.arizona.edu/pubs/insects/az1046/
published August 1998
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