Alfalfa Weevil

 

https://entomology.k-state.edu/extension/insect-information/crop-pests/alfalfa/

alfalfa-weevil.html

An invasive insect is lurking in Alfalfa fields across the Midwest waiting to make itself known each spring. Alfalfa Weevils are believed to originate in Asian countries. They eventually made their way to Europe, where in turn they made their way to the United States in 1904 when they were first discovered in Utah.

Light brown with a dark stripe down their back, measuring approximately ¼ of an inch in length with a pronounced snout these small beetles are rather drab and unassuming. What they lack in the looks and size department, they more than make up for in the appetite department. Adults spend the winter months hidden within the stems of the alfalfa plant, or among leaf litter and other debris. In early spring they become active and egg laying begins. Eggs are laid within the stems of newly developing plants. The larva will make their way to the leaves where they will begin feeding. Newly hatched larvae leave behind tiny pin-hole size feeding evidence that is noticed as the leaves begin to unfurl. As the larva grow, so too does the size of the feeding holes. If the population of larvae is too excessive the leaves will take on a skeletonized appearance and excessively infested fields may look gray or whiteish from a distance. 

https://agcrops.osu.edu/newsletter/corn-newsletter/2018-09/when-begin-alfalfa-weevil-scouting

Another hay crop pest called the Clover Leaf Weevil is very similar in appearance and often shares the same fields with the Alfalfa weevil. To determine which weevil is feeding on your crop, pay close attention to the larvae. The CLW will have a brown head, whereas the AW has a black head. Nearly all the damage done to the annual alfalfa crop is done by the larvae. Adults are not known to cause any significant damage as they typically feed on the edges of the leaves near the bottom of the plants. Although exceptions exist. If the adults begin feeding on the newly developing plant stems after the first cutting, this may cause something known as Bark Feeding and cause damage to newly developing stems.

https://ohioline.osu.edu/factsheet/ENT-32

 With an estimated 15% yield loss on heavily infested fields, typically in central or southern Missouri, some control is needed. Early harvesting can control weevil populations, as too can grazing cattle on infested fields. The estimate is as much as 90% eradication of the weevils in grazed fields. Paying close attention to fields and determining if your field is past a sustainable threshold of weevils is key to controlling them. Pick several stems from various locations within the field, place them in a bucket and shake them. Count larvae and adults. If the number exceeds more than 1 weevil per stem, then chemical intervention may be warranted. It should, however, always be the last resort, not the first option.

 We should always be mindful when making the decision to utilize chemical control of pests. Over usage of chemicals can and will create super pests. The premise is that approximately 5% of pests will survive the chemical onslaught, these nearly 5% will carry a certain amount of resistance into the next generation. Within a few generations most of the pests you are targeting will have some if not complete resistance to the chemicals you are using. Chemicals are not species specific in their target. Meaning, when you spray chemicals you aren’t just killing your desired target, but also beneficial insects, such as lady beetles, lacewings, damsel bugs and other predators of the pest you are trying to eliminate. You are also putting at risk other beneficial insects such as pollinators.

 Numerous predatory wasps have been introduced throughout the United States from Asia and other countries where the Alfalfa weevil originated. These predators evolved with the weevil and provide excellent population control in their native home. Several of these predatory wasps are now well established in the United States and may control as much as 35% of weevil populations in each location. If you have several wasp species within the same field all the better for control. Lady Beetles, Lacewings, and Damsel Bugs are all predatory at some stage in their lives and do an excellent job of controlling pest insects. As too does birds, bats and small mammals. 

https://commons.wikimedia.org/wiki/File:Hypera_postica_P1650452a.jpg

 

During springs when the weather is wet and warm a naturally occurring fungus consumes the weevils, killing as much as 90% of their population. Slow moving weevils that range in color from pale yellow to black are a strong indication this fungus is at play. Within a few days infected weevils will perish.

Being vigilant and keeping a close watch on your fields in early spring will go a long way in making the appropriate decision about control measures of this invasive, and well-established hay pest. Further questions or concerns should always be discussed with your local extension office. They are equipped to offer the best advice for your situation.