Wondering what critter ate your timothy this spring? In addition to armyworm, there was also some European skipper (Thymelicus lineola) damage to timothy and other forage grasses. While it is typically fairly isolated, in 2012, the “year of the forage insect”, we had an unusually high number of reports. Damage and larvae were confused with armyworm. Larvae feed on leaves, notching them irregularly and eventually defoliating them. With very high populations, they will also feed on the heads, leaving only the stems remaining.
This pest was introduced to the London Ontario dump in 1910 in swamp grass used to pack glassware shipped from England. It is usually held in check by a naturally occurring fungus disease, but occasionally there are some local outbreaks.
Younger larvae have black heads that eventually turn brown. Mature larvae are light green, ¾ inch (19 mm) long, with brown heads and two light bands. Larvae can usually be found within leaves, that they roll and attach with silk webbing, where they feed. The adult is a pumpkin-orange butterfly with a ¾ inch (19 mm) wing spread that “skips” about hay fields from mid-June to mid-July.
Eggs over-winter on the stems of crop debris and hatch in the spring. Larvae crawl to the top of the plant, feed on the flag leaf, and then work down the stem. Larvae feed on timothy until late-June. The larvae then attach themselves to grass stems or weeds and develop into chrysalids (the pupa stage of the butterfly). The adult skipper emerges in 2 weeks and feeds on the nectar of flowers, doing no further damage. There is one generation per year.
Scouting & Control
Begin scouting for larvae by late-April. Remove five random, one-square-foot (30 cm2) samples of forage down to ground level and place them along with the old crop residue into a bag. Tie the bag and leave overnight at room temperature. The caterpillars will crawl out of the residue and can be counted easily.
If 6-8 larvae in a 30 cm x 30 cm area are found and larvae are still small (brown-headed stage), treat the field or the infested area. Products containing Bacillus thuringiensis (Bt) are available. Refer to OMAFRA Publication 812, Field Crop Protection Guide http://www.omafra.gov.on.ca/english/crops/pub812/3skppr.htm.