Fall Armyworm Alert – Forages, Corn, Cereals and Others at Risk

Several reports of fields with fall armyworm (FAW) larvae have been coming in over the last few days.  Growers or scouts drive by a field and pull over, shocked to see only stems left of plants with larvae happily feeding. So far most of the damage has been found in oats, rye cover crop, mixed forages and newly seeded cereals. This pest overwinters in the southern US, Mexico and Central and South America. Moths migrates up to this area in late summer and are typically light or arrive too late to be of concern. Not this year, apparently. The Great Lakes Region and other states are finding significant populations of larvae in some host crops. Damage is usually not spotted until the larvae are larger and are in their most hungry stages (4th – 6th instars). Unfortunately from the calls so far in Ontario, the larvae have been too big (larger than 1 inch) for insecticides to be effective but thankfully these larger larvae won’t be feeding much longer before they start to pupate.

Fall armyworm vary in colour from tan to almost black and have three white stripes along their back. If you look closely, you can see hairs coming out of “warts” along their body and they have an inverted white Y on their black “face” (Figure 1).  Fall armyworm feed on grassy crops just like true armyworm do but FAW can also feed on some broadleaf plants like alfalfa and clover and some weeds like pigweed and fleabane (could they just stick to the weeds please?). Similar to true armyworm, they can do a lot of feeding in just a matter of days, marching from host field to host field.

Scouting is advised for any of their potential host crops to determine if they are present including corn, sorghum, cereals, mixed forages and cereal cover crops. The younger the crop, the more appealing it likely was for moths to lay eggs in.  FAW larvae can be found feeding during the day. Scout at least 10 areas of the field, first starting at the edge of the fields that may be adjacent to other host crops. Patches of thinning stand is also a potential sign of feeding activity and should be investigated. Take note of the size of larvae that are found. If most of the larvae are 1 inch or larger, it is too late to control them but early harvest can save some crops from further damage. Larvae this large will be stop feeding soon and will go into pupation.