The Alfalfa Weevil Are You Doing Battle with this Alfalfa Enemy

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					                                  The Alfalfa Weevil
                     Are You Doing Battle with this Alfalfa Enemy?

The Alfalfa Weevil is a tiny pest that can cause BIG losses to your crops. Management of this
pest is essential to reduce crop losses, particularly during years when weevil infestation is
high. It is necessary for growers to become familiar with sampling procedures, management
guidelines, and control recommendations so control techniques are not used unnecessarily.
Below are several articles to help you learn all you can about this insect.
    Alfalfa Weevil in the Midwest: A Successful Case of Classical Biological Control

The alfalfa weevil, Hypera postica, is an example of a highly successful case of the
"classical" approach to biological control in the Midwest. The alfalfa weevil is known as a
serious pest of alfalfa in the Midwest and throughout the U.S. The weevil damages alfalfa by
chewing and skeletonizing the foliage. Both adults and larvae feed on the plant, but the
larvae are by far the more injurious stage. The alfalfa weevil typically has just one
generation per year, with larvae present during the spring, and thus the weevil is a pest
mainly of the first alfalfa crop. However, following the first cutting, surviving larvae and
adults can significantly delay alfalfa regrowth by feeding on the new growth of the second

In the Midwest, alfalfa weevil adults emerge from pupae during late spring or early summer,
feed for several weeks, and then spend the remainder of the summer in a quiescent state
known as "aestivation." Aestivation is completed by late summer or fall, and the adults
become active, feed and begin to lay eggs during the fall (Fig. 1). Fall oviposition is
substantial in the southern parts of the region but is usually inconsequential in the north.
Both eggs and adults are capable of overwintering.

 Figure 1. Alfalfa weevil seasonal history.
 Figure courtesy of

Both the alfalfa plant and the alfalfa weevil are native to the Near East and Central Asia.
Alfalfa was introduced into the Midwest (Minnesota) in the 1850's. Alfalfa weevil
populations were accidentally introduced into the U.S. three times: the "western strain" of
the alfalfa weevil into Utah in 1904, the "Egyptian" alfalfa weevil into Arizona in 1939, and
the "eastern strain" of the alfalfa weevil into Maryland in 1951. (Egyptian alfalfa weevil is
actually a different species, H. brunneipennis, but some authorities now think it and the
eastern strain are more closely related than are the eastern and western strains.) It is
primarily the eastern strain weevil that occurs in the Midwest, although western strain
weevils can be found on the western fringe of the region. This report will focus on biological
control efforts against the eastern strain.

As an exotic pest, the alfalfa weevil was considered a good candidate for importation
biological control, and a program of introducing natural enemies against the eastern strain
weevil was initiated by the Agricultural Research Service of the U.S. Department of
Agriculture (USDA) in 1957. A number of parasitoid species (parasitic wasps) were
purposely introduced from Europe and the Middle East and released in the eastern U.S., of
which six became established. However, of these only one, Bathyplectes curculionis, was
able to move effectively with the weevil as it spread westward from the eastern seaboard. A
second alfalfa weevil biological control program was thus initiated in 1980, this time by the
Animal and Plant Health Inspection Service of the USDA, to redistribute the established
parasitoid species throughout the eastern U.S. This second phase lasted until 1988, and
resulted in five parasitoid species becoming widely established in the Midwest (Table 1).

 Table 1. Alfalfa weevil natural enemies established in the Midwest
 Name                       Type             Weevil Stage Attacked/Killed
 Anaphes luna               parasitic wasp   egg/egg
 Tetrastichus incertus      parasitic wasp   larva/larva
 Bathyplectes curculionis   parasitic wasp   larva/prepupa
 Bathyplectes anurus        parasitic wasp   larva/prepupa
 Erynia phytonomi           fungal pathogen larva & pupa/larva & pupa
 Microctonus colesi         parasitic wasp   larva/adult
 Microctonus aethiopoides   parasitic wasp   adult/adult

Included also in Table 1 are two natural enemies that occur in the Midwest but were not
introduced through the USDA programs. Anaphes (=Patasson) luna, an egg parasitoid, was
probably accidentally introduced into the eastern U.S. Erynia (= Zoophthora) phytonomi, a
fungal pathogen that infects and kills alfalfa weevil larvae and pupae, is a more interesting
story. This fungus first appeared in Ontario, Canada in 1973 and now commonly occurs with
the weevil throughout the Midwest, but it is not known to occur outside North America.
Although there are several theories about where the fungus might have come from, its origin
remains a mystery.

Impact on Alfalfa Weevil Populations. A problem many classical biological control
programs share is lack of follow-up analysis to rigorously assess the impact of newly
established natural enemies on the pest and thus document the success or failure of the
program. This has not been the case with alfalfa weevil biological control in the Midwest.
Two excellent studies have provided information on and insight into the impact natural
enemies are having on alfalfa weevil populations in the Midwest.

The first study was a series of alfalfa weevil life tables initiated in Ontario, Canada in the
early 1970's. Although this study was not initially intended to document the impact of
natural enemies on weevil populations, during the course of the study Erynia phytonomi
appeared for the first time, and several parasitoids became established in Ontario as a result
of the USDA programs. Thus, this study was able to examine alfalfa weevil population
dynamics "before and after" the establishment of natural enemies. Among other things,
analyses of the life tables demonstrated that E. phytonomi and Microctonus aethiopoides
plus M. colesi were important regulating factors acting to suppress and stabilize weevil
populations at subeconomic levels, and also that sensitivity of the fungal pathogen to
environmental conditions (this pathogen, like most entomopathogenic fungi, needs a moist
environment for spore germination and infection of host larvae) can lead to reduced
effectiveness of the pathogen in dry years, which destabilizes the system and can lead to
weevil outbreaks.

The second study was conducted specifically to assess the impact of the USDA parasitoid
redistribution program on alfalfa weevil populations and the economics of alfalfa
production. This analysis indicated that, by any measure, the program was a success.
Following redistribution, parasitism of larvae (primarily by B. anurus) and adults (by M.
aethiopoides) increased, alfalfa weevil population densities declined, and fewer alfalfa fields
were sprayed with insecticide for alfalfa weevil control. Moreover, a detailed economic
evaluation concluded that the benefit:cost ratio accruing from the biological control program
was an almost unthinkable 91:1!

In addition to these studies, data on parasitism and disease in alfalfa weevil populations from
various states in the region have indicated that in terms of magnitude or abundance, the most
important alfalfa weevil natural enemies are the two Bathyplectes spp., E. phytonomi, and
M. aethiopoides. Several years ago when I was preparing a presentation on biological
control in alfalfa, I conducted an informal poll of alfalfa research and extension
entomologists in the Midwest to determine their views on alfalfa weevil biological control,
and I received the following responses.

     Q: How do you rate biological control of alfalfa weevil in your state (complete,
     substantial, or partial)?

     A: Complete (1), Substantial (5), Partial (2)

     Q: Rank the following for their contribution to alfalfa weevil control in your
     state: Bathyplectes curculionis, B. anurus, Erynia phytonomi, Microctonus
     aethiopoides, and other natural enemies.

     A: No. 1 rank: Microctonus (4), Erynia (3), and a 3-way tie between Erynia,
     Microctonus and B. anurus.
Clearly, the consensus view in the Midwest is that biological control is an important
component in the management of the alfalfa weevil. Interestingly, the species perceived to
be the dominant natural enemy tended to vary geographically, which is perhaps a reflection
of climatic differences. Microctonus tended to be rated the most effective in the northern and
eastern parts of the region, whereas Erynia tended to be rated most effective in the western
and southern parts.

Finally, why has biological control of the alfalfa weevil proven to be such a success in the
Midwest? I think attributes of the crop, the pest and the natural enemies have all contributed
to this.

Crop: Alfalfa is a perennial and thus provides a relatively stable habitat for natural enemies.
In addition, flowering weeds in the crop may provide a nectar source for parasitoid adults,
and alfalfa fields are generally treated with insecticide infrequently if at all. Although
frequent harvesting of the crop can be destabilizing, the successful natural enemies are not
unusually harmed by this practice (see below).

Pest: Life history characteristics of the alfalfa weevil --narrow host range (essentially only
alfalfa in North America), limited dispersal ability, and long generation time-- suggests that
this species should be vulnerable to attack by specialist natural enemies and thus well suited
for biological control.

Natural Enemies: The natural enemies that have proven to be important in alfalfa weevil
biological control share two attributes that have been keys to their success. First, their
seasonal histories are well synchronized with the alfalfa weevil seasonal history. Second, the
successful natural enemies tend to be "resistant" to harmful effects of alfalfa harvesting.

- Dave Hogg, University of Wisconsin-Madison
Scout first and then decide: Using economic thresholds for
alfalfa weevils

 by Marlin E. Rice, Department of Entomology

 Scouting for alfalfa weevil larvae is not that difficult, although it takes several minutes when done properly.
 In any alfalfa field, I prefer to start my scouting with a sweep net. A sweep net and a quick and easy
 method of answering the question, "are there alfalfa weevil larvae in the field?" By taking 40-50 sweeps in
 an area, it can easily be determined if larvae are present or not. If larvae are found, then proceed to the
 stem-sampling technique, which is used to determine the economic threshold. Stem sampling can more
 accurately determine the population size and the potential for yield loss.

 Collect 30 stems by holding the top of the plant with one hand and breaking the base of the stem with the
 other hand, or cut it with a knife. By holding the top of the stem, this prevents larvae from dropping from the
 stem when it is broken or cut at the base. Then place the stems (upside down) inside a white, 5-gallon
 bucket and beat them against the side. Large larvae are easily knocked loose and can be counted, but
 newly developing leaves in the terminal must be pulled apart to find very small, newly hatched larvae
 hidden in the plant tip.

 Alfalfa weevil larvae can be recognized by a very dark head, which is almost black, and a pale green with a
 white stripe along the back. When the larvae hatch, they are approximately 1/16 inch in length and may be
 light yellow in color. After feeding for several days, they turn green. They are 5/16 inch in length when fully

 Alfalfa weevil larva and damage to alfalfa leaf. (Marlin E. Rice)

 Alfalfa weevil larvae may be confused with larvae of the clover leaf weevil, although these are much larger,
 have a light brown head, and often have the white stripe edged with pink. Clover leaf weevil larvae usually
 hide around the base of the plant during the day, feed mostly in lower leaves at night, and rarely cause
 economic yield losses. Clover leaf weevil larvae should not be counted as part of the alfalfa weevil sample.
The economic threshold will go up if diseased larvae are found in the field. (Marlin E. Rice)

Measure the plant height and then determine the average number of weevil larvae per stem, based upon a
30-stem count, and then consult Table 1 for the economic thresholds. The economic threshold depends on
crop height, estimated crop value, control costs, and the growing conditions stated in Table 1. Use the
smaller threshold if alfalfa is drought-stressed, or control costs are relatively low ($7-10 per acre). Use the
larger threshold if rainfall is abundant, diseased larvae are present, or control costs are relatively high ($11-
14 per acre). Several commonly available insecticides labeled for alfalfa weevils are listed in Table 2.

Table 1. Economic thresholds based on alfalfa weevil larvae per stem, calculated from a 30-stem

Height       $40/ton    $70/ton    $100/ton    Management Decision
                                                Reevaluate in 4 days. If damage and larval numbers are
4            1.8-2.8    0.8-1.3    0.6-0.8      increasing, a long-residual insecticide is recommended to
                                                prevent severe yield loss.
6            2.0-3.0    0.8-1.5    0.6-1.0
8            2.2-3.2    0.9-1.7    0.7-1.2
                                                If alfalfa is in vegetative stages, a short residual insecticide
10           2.3-3.5    0.9-1.9    0.8-1.4
                                                should be used.
12           2.4-3.8    1.0-2.2    0.9-1.6
14           2.5-4.2    1.2-2.5    1.0-1.8
                                                If >60 percent of alfalfa is in the bud stage, harvest is
                                                recommended. Evaluate stubble after harvest. If not
16           2.6-4.6    1.5-2.8    1.1-2.0
                                                scheduled to be cut within 7-10 days, a short-residual
                                                insecticide is recommended
18           2.7-5.0    1.7-3.1    1.2-2.3
20            2.8-5.8     2.0-3.4   1.4-2.6
>20           3.0-7.0     2.4-4.0   1.6-3.0

Use the smaller threshold if alfalfa is drought-stressed, or control costs are relatively low ($7-10 per acre).
Use a larger threshold if rainfall is abundant, diseased larvae are present, or control costs are relatively
high ($11-14 per acre).

Table 2. Commonly available insecticides labeled for alfalfa weevil.

                 Rate per Acre
Insecticide                                   Harvest Interval (Days)
                 (High and Low Rates)
Baythroid 2E      1.6-2.8 ounces              7
Furadan 4F        0.5-2 pints                 7-28
Lannate LV        3 pints                     0
Lorsban 4E        1-2 pints                   14-21
Mustang Max       2.24-4.0 ounces             3
Pounce 3.2EC 4-8 ounces                       0-14
Sevin XLR+        3 pints                     7
Warrior           2.56-3.84 ounces            7

This article originally appeared on pages 104-105 of the IC-498 (5) -- April 9, 2007 issue.
Mistaken identity.(pests)
Source:    Farm Journal
Publication Date: 15-MAR-04

Don't confuse clover leaf weevil for its more damaging cousin, the alfalfa weevil

When spring rears its head and degree-days start to accumulate, it's time to begin watching for alfalfa
weevils. Left unchecked, larvae can gorge on the leaves of first-growth alfalfa while adults further
skeletonize leaves and any regrowth.

As you begin scouting, be advised that the clover leaf weevil also may be lurking in your fields, says Kelly
Cook, University of Illinois Extension entomologist. "These two pests commonly cause confusion in
identification during scouting," Cook adds. "The alfalfa weevil and clover leaf weevil are similar in
appearance, but can be distinguished on closer inspection."

It's important to differentiate between the two pests to correctly assess the problems at hand and to
avoid spraying insecticides when they are not needed, Cook says.

At the larvae stage, good scouting requires an eye for distinguishable patterns and markings between the

Pale green in color, the clover leaf weevil larva has a white stripe down its back and a tan head. As it
gets older, a pink border surrounds the white stripe. The alfalfa weevil is a smaller larva that is also pale
green with a white stripe, but it has a dark brown or black head.

Clover leaf weevil larvae feed on the plant's lower leaves, primarily at night, and will be in the debris
around the crowns during the day. "Scratching in the soil around the crowns and counting the number of
larvae found per crown will give an idea of clover leaf weevil infestation," says Keith Jarvi, University of
Nebraska integrated pest management specialist.

Alfalfa weevil larvae stay on the plant and feed at or near the top.

As the weevils grow into adults, their colors change. An adult clover leaf weevil is brown with a wide,
dark brown stripe on its back. The alfalfa weevil is about half the size of a clover zleaf adult and is brown,
with a dark, narrow stripe along the center of its wing covers.

Typically, clover leaf weevils are only a problem in dry springs. Also, these pests are susceptible to fungal
diseases, which keep them in check.

If clover leaf weevils are found, farmers shouldn't be too concerned, Cook says. The clover leaf weevil
usually doesn't cause the same severe, first-crop yield reductions that occur from alfalfa weevil feedings.

Scout effectively. Scouting for weevils should begin three to four weeks before the first cutting, says Rod
King, Golden Harvest technical service agronomist. When you first scout for weevils, use a sweep net to
determine if any larvae have hatched.

If larvae are found, collect 20 stems from each of five places in the field and look for larvae in the upper
leaves. Hold stems upside down in a bucket and shake vigorously to dislodge larvae into the bucket,
where they can be counted easily.

"I recommend treatment when about 40% of the stems show tip feeding and three or more weevil larvae
per stem are found," King says. There are several insecticides for weevils, ranging in cost from $7 to $12
per acre.

If your fields show high levels of alfalfa weevils or clover leaf weevils within seven to 10 days of cutting,
King says it may be more economical to harvest the alfalfa. But then watch regrowth for any weevil

Weevil Comparison
Alfalfa Weevil

Overwinters primarily as adult

Adults are brown with dark brown stripe halfway down back, 3/16" long

Larvae prefer to feed on tips

Larvae remain on plant most of time

Larvae have black heads

Adults leave fields in June

Adults may remain in fields

Clover Leaf Weevil

Overwinters as larva

Adults are dark brown, pitted light brown underneath, over 1/4" long

Larvae feed anywhere on plant

Many larvae are in soil or debris during the daytime

Larvae have brown heads
                                                                          Alfalfa Weevil Activity
      Tracey Baute, Field Crop Entomologist, OMAFRA, Ridgetown
                  Joel Bagg, Forage Specialist, OMAFRA, Lindsay

Scouts are starting to notice 1st and 2nd instar alfalfa     If there is 40% leaftip feeding, with 2 or 3 active
weevil larvae in their sweep samples. That tells us that     weevils per stem, and there is more than 7–10 days
we will have the larger sized (and hungrier) 3rd and 4th     to preferred harvest date, consider applying an
instars larvae in a matter of days to weeks (weather         insecticide. Less than 1 active larva per stem does not
dependant). We could get lucky with this wet weather         require action, but continue to monitor the situation.
and see a fungal pathogen start to kill these larvae but     Two larvae per stem requires action if the alfalfa is less
we can’t count on it. Fields                                                                   than 40 cm (16 in.)
should be scouted twice a                                                                      high. If there are more
week for the next month to                                                                     than 3 active larvae per
stay ahead of any injury.                                                                      stem, immediate action
Larval activity tends to start                                                                 is required.
first on the shallow soils or
on southerly slopes of the                                                                Cutting early is the
fields. Peak activity usually                                                             recommended
coincides with the bud                                                                    management strategy.
stage of the first crop.                                                                  Insecticides are only
                                                                                          recommended when
To scout for larvae, collect                                                              cutting is impractical,
30 stems in a large M                                                                     i.e. when the alfalfa is
shaped pattern across the                                                                 in the pre-bud stage.
field.                                                                                    For a list of
Place them inside a white pail     Figure 1. Third and Fourth Instar Alfalfa        recommended products, refer
and beat the plants against the    Weevil Larvae. (T. Baute, OMAFRA)                to the OMAFRA publication
side of the pail to knock off the                                                   812, Field Crop Protection
larvae. Check the plants for any remaining larvae that      Guide at:
did not drop into the pail before tossing the plants aside. pub812/4weevil.htm
Also make note of any leaf tip feeding on these plants.
Leaf-tip feeding refers to the percent of plant tips
showing obvious signs of damage, which is not to be                     Mark Your Calendar!
confused with the percent defoliation.

The first and second instar larvae that fell into the pail
                                                                     Wednesday, July 8, 2009
are not as important and should not be counted. They                           Or
are the larvae that are only 3 mm or less in size and are             Thursday, July 9, 2009
still pale yellow to light green, with not much of a white
stripe to see. Once they are in the third and forth                       15th Annual
instars, they are bigger in size (up to 8 mm) and are a           Southwest Crop Diagnostic Day
pale to bright green, with a black head (Figure 1). Once
they are this big you can easily see the white stripe down                                Visit the Website
the back. These are the ones that you do want to count                        
in the pale. Only count those 3rd and 4th instar larvae
that appear healthy and are actively moving around.                                   ....more details to follow in
Those that are infected with fungus will be slow moving,
                                                                                       future issues of CropPest
yellow or tan and should not be counted.
                                                                                CropPest Ontario .   May 14, 2009


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