True Armyworm 2. Update August 28, 2017

Moth traps were set up during early August near fields that experienced damage during the first generation of caterpillars/armyworms. Moths have been seen in fields and caught consistently in traps in Port Alberni, Courtenay/Comox, and Cowichan valley locations. Traps are checked approximately weekly and number of moths recorded.

moth trap 

moth

 

moth

It is time to scout for armyworm larvae (caterpillars) in the lush regrowth of the grass fields. Check in areas near or adjacent to the earlier armyworm-damaged fields. Moths will be looking for lush green growing grass to lay new eggs in August. Larvae will be feeding now in those areas. Scout by laying down a ruler or meter stick, and estimating a square foot or quarter meter square and count the larvae you see feeding in the grass. You will need to be down on your knees where you can look closely for larvae up to 1.5 cm long (3/4 inch). Do the first count this week, and then count again in one week. If, on average, more than 5 larvae per square foot are found, you can expect damage from feeding to the grass crop. Do counts in several locations (at least 20) in a field each time before deciding if and where to treat. It may be that only edges or part of a field need to be treated, as the population may be localized or spotty. Armyworm larvae may be green or darker, and have a brown or mottled head capsule. We do not expect to see as many as in July, but is worth counting larvae in areas of concern to know for sure what is happening in the fields.

 larva
Full grown larva

larva
Close up of netted pattern and "V" on head of larva.

pupa
Armyworm pupa in soil. 

 

grass damage
Use a ruler, measuring tape, or yard stick, and count the larvae in 1 square foot.  This photo is from the first generation, when there were many larvae per 1 sq.ft. (and lots of damage).

 grass damage
Look for feeding damage in grass.  This is fairly significant feeding by caterpillars during July (first generation of larvae).

Larvae may also feed on corn, but the corn will be able to withstand some feeding and no control actions are expected to be necessary.

corn damage
Larval feeding damage and their frass pellets on young corn plants (July).

If you meet or exceed the larval threshold for grass crops, control may be warranted.

Options for your consideration:

  • Foliar spray if possible or desired
  • Cut hay early to minimize damage (get the crop before the caterpillars do)
  • Graze sooner rather than later (caterpillars are not hazardous to livestock)

More information:

 

Prepared by: Tracy Hueppelsheuser, B.C. Ministry of Agriculture, Tracy.Hueppelsheuser@gov.bc.ca 604-556-3031