MSU Weed Science - Department of Crop and Soil Sciences
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Nov 23rd, 2017

INSECT PESTS of ALFALFA
in MICHIGAN


Potato leafhopper
Alfalfa weevil
Winter cutworm
Spittlebug
Blister beetle



Recommendations for control of insects in alfalfa and hay in Michigan are published annually in the MSU bulletin E-1582 Insect, Nematode, and Disease Control in Michigan Field Crops. This bulletin not only lists insecticide recommendations and rates, but provides a summary of biology, sampling methods, threshold, and IPM practices for each insect. Click on the link below to access individual chapters for alfalfa and hay.

E1582 chapter for alfalfa insect pests
E1582 chapter for grass hay insect pests


Potato leafhopper

is the most important and yield-robbing insect pest of alfalfa in Michigan. Its tiny size creates the impression that it can't be doing very much in the field. However, both nymph and adult PLH secrete a salivary toxin as they feed which injures the plant beyond simple removal of plant juices. The classic symptom of PLH feeding is hopperburn (leaf yellowing) as well as leaf crinkling. But by the time symptoms are apparent, considerable damage has already been done. Therefore it is very important to monitor PLH numbers, then properly time cutting or spray applications. PLH is an example of an insect where scouting pays. An alfalfa field correctly managed for PLH should have little leaf yellowing and crinkling.

For more information on potato leafhopper
Potato leafhopper in Michigan soybean and dry bean (pdf)


Alfalfa weevil

is making a comeback as an alfalfa defoliator. An introduced non-native pest, alfalfa weevil was found in the eastern U.S. in the 1950s. Although insecticides were first used to manage the weevil, a successful biological control program found and introduced several parasitic wasp species that kept the weevil population below damaging levels for many years. In the last decade, however, weevil numbers are on the rise in many states in the Midwest and defoliation is more common. It is not clear why populations have increased, but insecticide applications are more common. As insecticide use increases, the impact of biological control is  likely further disrupted, continuing to exacerbate the problem. It is thus critical to manage alfalfa weevil with cultural methods (such a cutting) first, and to only apply insecticides when weevil defoliation is over threshold.
Note that there are several species of weevils (or snout beetles) in alfalfa fields. In this picture, the alfalfa weevils are the smaller beetles, many of which could fit on a penny. The larger beetle is a less common species found sometimes in sweep nets.
 

 


Left ~ Alfalfa weevil larvae collected from 50 sweeps in a heavily infested field.
Right ~ Close up shows distinct black head capsule and white stripe along the back.


For more information on alfalfa weevil
Alfalfa weevil life cycle and control (pdf)

Winter cutworm (Noctua pronuba)

is recent pest of alfalfa in Michigan. Winter cutworm caterpillars feed on alfalfa and small grains in the fall. Fall defoliation of alfalfa after a hard freeze does not seem to impact root reserves, but may remove plant residue that protects crowns from winter kill. Fall defoliation of small grains and grass hay can kill the field.

Winter cutworm defoliation of alfalfa (left). 
Caterpillars trapped on a cold night on an Ag-Bag covered alfalfa bale. (right)



One of the key traits of winter cutworm is its  incredible cold tolerance. Caterpillars continue to feed in the middle of winter on sunny days. This winter cutworm was found crawling across the snow surface in February in northern Michigan, when air temperature was in the low 20s.



For more information winter cutworm

Winter cutworm: First report of economic damage in Michigan (pdf)


Spittlebugs

are usually noticed because of spit globules hanging on alfalfa stems. Inside a spittle mass is one or more spittlebug nymphs. They have devised an interesting and effective way to avoid being eaten - they secrete a frothy liquid from their anus. I think we can agree that butt-froth is a deterrent. Spittlebug adults and nymphs feed on plant juices with a piercing-sucking mouthparts. While curious, I have never seen large enough populations to justify spraying.

Spittlebug mass on alfalfa (left) and spittlebug nymphs (right).



Blister beetles

are an occasional pest, not for the minor feeding damage they do to alfalfa leaves and flowers, but for the potential for livestock poisoning. The beetles contain a compound called cantharidin, which blisters sensitive tissues such as the mouth and gut cattle, sheep, and  especially horses. When beetles are in large numbers in alfalfa at harvest, they are killed by equipment and contaminate baled hay. Blister beetle outbreaks often follow grasshopper outbreaks in dry years. This is because blister beetle larvae are actually beneficial predators of grasshopper eggs.

For more information on blister beetles
Cantharidin content and lethal dose of common Michigan blister beetles (pdf)


Gray Blister Beetle

Blister beetles are not only found on alfalfa. This specimen was on a flowering weed, feeding on pollen. Note the characteristic barrel-shaped body and its narrow 'neck' (just behind the head).

In addition to gray blister beetle, two other common species in Michigan are the black and striped blister beetles