Extension, College of Agriculture & Life Sciences, The University of
Tim C. Knowles, Area Extension Agent, La Paz and Mohave Counties
Description & Biology Of The Pest
The first sign of a potential alfalfa caterpillar (Colias eurytheme)
outbreak is the influx of large numbers of yellow or white butterflies
in late spring or early summer. The butterfly reaches 1 inch long with
a 2 inch wingspan. Its wings are yellow, orange, or white with a black
border on the upper surface and are solid white or yellow on the underside.
When butterflies are seen flying over a field of tall alfalfa, they
have probably just emerged from that field. They will remain to feed
on flowers and mate, but the females will move into alfalfa less than
6 inches tall to lay eggs. White 1/16 inch long eggs are laid singly,
standing on end on the upper leaf surface, and hatch into small ½
inch long, brown larvae with black heads in 3 to 10 days.
The alfalfa caterpillar soon turns green and grows rapidly to 1 ½
inches long in approximately 2 weeks, depending on the temperature.
Seven generations occur between May and October. Older, larger caterpillars
have a prominent white stripe which runs along each side of the body.
The alfalfa caterpillar can be distinguished from the beet armyworm
by its hairy or velvety green body surface. Viewed with a hand lens,
the beet armyworm has a smooth body. The alfalfa caterpillar pupa hangs
from alfalfa stems by silken threads or rests on leaves. The caterpillar
overwinters as a pupa which changes from a gray green to yellow color
as it transforms into the butterfly that emerges the following spring.
The life cycle of the alfalfa caterpillar is closely synchronized with
the hay cutting cycle, taking about one cutting cycle to complete. Four
to seven generations occur between May and October.
Alfalfa caterpillars consume large portions of foliage or entire leaves.
Larger larvae are the most destructive. In contrast to beet armyworms,
alfalfa caterpillars do not skeletonize leaves and will consume the
leaf midrib, nor do they cause flagging in which alfalfa terminals dry
up and turn whitish-grey in color. Alfalfa caterpillar feeding damage
is most severe when eggs are laid in recently cut fields, and large
populations of developing larvae defoliate the crop before it reaches
maturity. When infestations occur in half to full grown plants, growers
can harvest early to avoid serious damage.
And Cultural Controls
and encourage natural enemies of the alfalfa caterpillar by avoiding
unnecessary insecticide applications. General predators including damsel
bugs (Nabis spp.), big eyed bugs (Geocoris spp.), and spiders will consume
eggs and larvae of the alfalfa caterpillar. The most important natural
enemies of the alfalfa caterpillar are naturally occurring insect pathogens
and parasitic wasps including Trichogramma semifumatum and Cotesia medicaginus.
C. medicaginis is a dark brown to black wasp about 1/4 inch long that
oviposits eggs inside early instar caterpillars. The egg hatches, and
the wasp larvae consume the body contents of the
caterpillar before it reaches ½ inch long. Parasitized larvae
are lighter in color, somewhat shiny lacking their velvety body surface,
and swollen toward the rear. If the parasitized caterpillar is pulled
apart at this swollen region, the shiny white parasite larvae is exposed.
Naturally occurring insect pathogens of the alfalfa caterpillar include
the bacteria Bacillus thuringiensis (Bt) and a virus. Dead bodies of
caterpillars infected with either disease are dark colored, and limp,
often hanging from leaves. Bt insecticide can give satisfactory control
of the alfalfa caterpillar, has no effect on beneficial insects, and
has a 0 day post harvest interval. Upon ingesting Bt, the caterpillars
cease feeding but may remain on plants 3 to 4 days before dying. Bt
application does not provide satisfactory control of the beet armyworm.
Effective cultural control can be achieved by cutting the crop early
to avoid serious damage, yet achieving acceptable hay yields.
Monitoring And When To Treat
When yellow and white alfalfa butterflies first appear in May, start
checking fields for alfalfa butterflies. Large numbers of yellow or
white butterflies flying above short alfalfa provides a good warning
signal for possible problems later in the cutting cycle. Take sweep
net samples starting when the alfalfa is half grown (more than 6 inches
tall). Monitor fields weekly from June through September. Make 5 sweep
counts at four or five locations in the field, noting the percentage
of parasitized and diseased caterpillars. Check two or three times a
week if heavy populations begin to develop. Check caterpillars for parasitism
by pulling heads off larvae, squeezing out the body contents, and looking
for Apanteles larvae. Treat for alfalfa caterpillars when 5 to 10 non-parasitized
or disease free caterpillars are collected per 90 degree sweep (or 10
per 180 degree sweep) more than 1 week prior to cutting alfalfa. Once
alfalfa enters summer slump, and hay tonnage and quality decline, growers
may not be able to justify the cost of insecticide applications for
Currently Registered Insecticides for Altalfa Caterpillar in Arizona
|Bacillus thuringiensis, Bt
(Biobit, DiPel, Javelin)
||Caterpillars cease feeding,
but may remain for 3-4 days
|3 days (4L, 50W, 80S)
7 days (Oil ULV)
|Do not apply to wet foliage
Apply only once per cutting
|½ pint = 7 days
1 pint = 14 days
2 pints = 21 days
|One application per cutting
Four applications per season
||One application per cutting
11.2 fluid oz/acre/season limit
||One application per cutting
0.96 fluid oz/acre/season limit
|0 days (EC)
7 days (Cythion)
|Do not apply when bees are working
||Ten applications per crop
(3.6 lbs. a.i./acre/crop).
||Do not apply during bloom.
Hazardous to bees.
|<0.1 lb. a.i. = 0 days
>0.1 lb. a.i. = 14 days
|0.2 lb. a.i./acre/ cutting limit.
Toxic to bees and fish (see label).
IPM Manual Group. 1985. Integrated pest management for alfalfa hay.
UC Davis Press.
Meister Publishing. 1997. Insect Control Guide. Wiloughby, OH
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.
The University of Arizona is an Equal Opportunity/Affirmative
Action Employer. Any products, services, or organizations that are mentioned,
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by the University of Arizona.
Document located http://ag.arizona.edu/pubs/insects/az1045/
Published August, 1998
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