Time to Survey for Alfalfa Weevil

Written by Kevin Wanner and Emily Glunk

Alfalfa weevil is the key insect pest of alfalfa, causing variable levels of economic damage across Montana each growing season. After mating the female weevils lay their eggs in alfalfa stems, and newly emerged larvae crawl up to the developing terminal buds where they chew small “pin” holes in the leaves. The larvae develop through four instar stages (Figure 1); the larger 3rd and 4th instar larvae feed openly on unfurled leaves and cause the largest economic loss. Severe feeding damage will give the field a “frosted” appearance. Mature larvae develop into the next generation of adults that leave the alfalfa field to find overwintering sites. In Montana there is one generation per year. The majority of crop damage occurs prior to the first cutting as a result of feeding by larger larvae. Management decisions are based on surveying the number of weevils to determine if their population will exceed the economic threshold, the point that warrants action to be taken.

Alfalfa weevil sampling should begin in the spring when the stand is about 8 to 10 inches tall. Weevil populations can be estimated using sweep nets (net with a 15 inch diameter, can be purchased online) or by shaking alfalfa plants in a bucket. An average of 20 alfalfa weevil larvae per sweep meets the economic threshold for action. Ten sweeps are taken at each of 3-5 five sites in a field (30-50 sweeps per field) and the total number of weevil larvae counted to determine the average per sweep. An alternative is to cut 10 stems from each of 3-5 different sites in a field (30-50 stems per field) and shake the stems in a bucket to collect the larvae. An average of 1.5 – 2.0 larvae per stem meets the economic threshold for action. To get an accurate average more samples are required for larger fields. A minimum of three samples are recommended for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. Based on historical weather data, sampling for alfalfa weevil in Montana typically begins between May 24 and June 16, depending on the location and the seasonal weather.

Typical dates that alfalfa weevil monitoring begins in Montana:

Sidney – May 24.    Glasgow – May 29.   Lewistown – June 13.   Kalispell – June 7.   Dillon – June 10.   Bozeman – June 8.   Red Lodge June 16.

When the economic threshold has been met (more than an average of 20 larvae per sweep or 1.5-2.0 larvae per stem) action is required to preserve yield. If stand growth is sufficient early harvesting is the most effective and economic action. If early harvesting is not an option then an insecticide can be used to reduce weevil populations below economically damaging levels. Additional management information including insecticide options is listed online in the High plains IPM guide: http://wiki.bugwood.org/HPIPM:Alfalfa_Weevil

Additional video resources:

Concerns When Feeding Frosted Alfalfa

Emily Glunk Montana State Forage ExtensionBy Emily Glunk, MSU Extension Forage Specialist

I have been getting many calls and emails from agents and producers about how to graze their frosted alfalfa. The biggest concern with grazing frosted alfalfa is the potential for bloat. Bloat is a serious problem in livestock, especially cattle, and preventative measures must be used when animals are placed in bloat-inducing situations, such as grazing alfalfa.

While a very nutritious forage, with high energy and protein values, grazing of fresh alfalfa comes with its risks. Typically, if a pasture is less than 50% alfalfa, there is a reduced occurrence of bloat. Care must always be taken when grazing alfalfa, even “non-bloating alfalfa”. “Non-bloating” or “bloat-safe” alfalfa have lower amounts of soluble proteins, the cause of bloat in ruminants. However, animals should still be monitored, because even though it is considered “safe”, bloat can still occur.

Why does alfalfa cause bloat in the first place? Soluble proteins in forages and other small particles within the cells of the plant are rapidly released once they reach the rumen. These proteins and particles are attacked by slime producing rumen microbes, which cause a buildup of stable foam. The foam decreases the animal’s ability to expel rumen gases that are created from fermentation of plant material. These gases begin to accumulate, causing pressure on the diaphragm, leading to bloat. In severe cases, the rumen can become distended, and death may occur.

montana alfalfa bloom feeding ranching hay cattleSo when does alfalfa become “safe” to graze? This seems to be the money question, as you will find several different answers. We know that we can feed pure alfalfa hay to ruminants, without causing any issues. This is because that forage has gone through a drying process, and the soluble proteins are significantly decreased. But at what point does it become safe, and what are some strategies that we can implement to decrease the risk of bloat?

Some things to consider are the environmental effects. Freezing of alfalfa, and grazing frosted/ frozen alfalfa, can significantly increase the chance of bloat. After a frost, the intercellular liquids freeze, and can puncture the cell walls, causing the cell to “burst” and contents to leak out. Soluble proteins will be released, and the incidence of bloat will be increased. If cattle are out grazing alfalfa during a frost, remove them immediately.

Some studies say that only three days are necessary after a frost to allow soluble proteins to decrease, however others cite that waiting five to seven days is safer. As a precaution, I generally recommend waiting about a week after a hard killing frost before grazing the alfalfa, at this point the plant has significantly dried down and the risk of bloat will be reduced.

Other recommendations for grazing frosted alfalfa include:

  • If it was not a killing frost, then wait until the alfalfa is in full bloom rather than bud to early bloom to graze. Soluble proteins decrease with increasing maturity.
  • Make sure that cattle are not turned onto alfalfa hungry. Feeding with a non-bloating forage beforehand will decrease the likelihood of bloat as they will not consume the alfalfa as rapidly
  • Monitor cattle for bloat several times throughout the day, especially when they begin to graze
  • Consider including the bloat preventative poloxalene (Bloat Guard) into your ration

Livestock that are suffering from bloat will begin to swell rapidly on the left side. If it is a severe case, the animal can die within the hour, which is why it is important to be constantly monitoring your animals. Kicking at their sides or stomping their feet are other signs that the animal is experiencing discomfort. If you notice any of your animals exhibiting these signs, make sure to call your veterinarian immediately.