Indoxacarb Helps Growers Escape Alfalfa Weevil But...
The warm spring gave alfalfa plants a head start in growth this season allowing growers to consider multiple options for weevil management. By the time alfalfa weevil larvae reached their threshold, alfalfa plants were within the harvest window. Some growers opted to harvest without an insecticide treatment since plants were able to withstand weevil feeding. In some cases, larval abundance was high and green-up after harvest was slow so insecticides were applied.
Growers have been searching for a replacement for the once available Furadan. Chemical management of alfalfa weevil has been limited to a few modes of action, organophosphates (group 1B) and pyrethroids (group 3), for which multiple products are available. Unfortunately, weevil suppression has come with mixed results using these products. This season several growers tried their hand at using indoxacarb (Steward) with a different mode of action for weevil management. Indoxacarb is categorized as a group 22 insecticide that works by blocking the sodium channel in the nerve axon and inhibiting a nerve impulse. A number of growers reported this product was successful for weevil management this season and it appears to be a product that may provide relief for growers battling weevil larvae every season.
This pesticide label for Steward (indoxacarb) shows the mode
of action (MOA) group number, which can be found on labels of
many newer products. Different active ingredients can share
a similar MOA.
One concern with indoxacarb’s success in Utah alfalfa is the impulse of many to rush to switch to one product and its eventual widespread use that could lead to problems with insect resistance over time. It is imperative that resistance management strategies be implemented early on to maintain and extend the life of such a successful tool. Monitoring weevil larvae and utilizing their thresholds to make a decision whether insecticide treatment is needed is the first step.
This season, even though weevil threshold may have been reached just before anticipated harvest, an early cutting was used in some instances and is recommended as an alternative to chemicals. Rotating among different chemicals with different modes of action can aid in slowing down insect resistance to chemicals when insecticides are chosen as the primary strategy for weevil management.
Product names and active ingredients may be different but they may share a similar mode of action. The Insecticide Resistance Action Committee (IRAC) has developed the classification of chemicals with different modes of action into different group numbers (click here for the pdf).
Chemical companies have made it easier to determine whether products have a similar mode of action by providing these group numbers on the label. Not all companies have followed suit in providing this information on the label so a search for the mode of action for the active ingredient will be required.
When using a chemical rotation, switching group numbers with successive applications is key. Alfalfa growers in Utah are able to rotate among groups 1, 3, and 22 for weevil management. Be aware that combination products containing two active ingredients with different modes of action are available but can limit options for chemical rotation.
-Ricardo Ramirez, Extension Entomologist