By: Wendy McDonald – MPSG Production Agronomist – West
No: After more scouting, as of that day, it seems this field that is was not at economic threshold for cutworms.
Scouting for Cutworms in Soybeans
threshold:
One or more Larvae per 3 feet of row and larvae are small (less than 2 cm)
or
20 per cent of Plants cut
We did more digging but didn’t come across any more clipped plants or cutworms. The cutworm we found was active but shy, with an interesting white bump protruding from his body. The white bump is a parasitic egg that will eventually kill this cutworm. Parasitic eggs on cutworms usually indicate the presence of natural beneficial insects, primarily braconid or tachinid wasps. These tiny wasps lay eggs inside a cutworm’s egg, and the wasp’s egg clones itself 100 or more times inside its host. The white larvae then eat the cutworm from the inside out. More than 100 wasps can emerge from a parasitized cutworm. Thanks Mother Nature!
In the same field, we did find evidence of another type of worm. This soybean seedling is showing evidence of wireworm feeding (a small hole in the seed which is surrounded by a browned and damaged looking area). This soybean plant will not develop normally and may not even reach the soil surface. When it comes to wireworms there are no post emergent insecticides that can deter or control them, only certain insecticide seed treatments could have helped with that.
Should we worried about more damage from cutworms occurring here?
Turns out this cutworm is either completely paralyzed or dead, thanks to the Fortenza seed treatment used on the soybeans in this field, providing residual cutworm protection. As the seeds germinated, the active ingredient (group 28, which is a non-neonicotinoid) was absorbed by the roots and moved within the growing soybean. Cutworms chewed on the new seedlings (that is why we found a few clipped plants), and the chemical they ingested messed with their muscle function. After a few bites, the cutworms became sleepy/ lethargic and eventually they will starve and die.
Faba beans and field peas can withstand some cutworm damage because they can regrow from sub surface nodes.
For more information on cutworm scouting in pulses and soybeans visit:
Cutworms in Pulse and Soybean Crops – Manitoba Pulse & Soybean Growers