WATCH OUT for Fall Armyworm

Contributed byRobert Moloney, NK Brand Seeds

Be on the lookout for Fall Armyworm.  We are seeing isolated infestations in a number of fields across the province this week.  The larvae are quite small which means that they are recently arrived and more fields with armyworm will likely show up.  There are a number of easy indicators that may tip you off that you have armyworm in a field.  The first is “notches” chewed out of the side of leaves.  These may be anywhere on the leaf, but can be concentrated in the whorl where the very small larva may be feeding.  The 2nd is the frass (brown, chewed up droppings) that the larva leave behind.  You will find these more often in or near the whorl, but they can be out on the leaf too.  The larva tend to hide in the ground under trash or down in the whorl of the plant during the heat of the day so they can be hard to find.

Armyworm damage on corn

Fall armyworm is a pest that doesn’t overwinter in Ontario, but blows in as moths from the south (as opposed to True Armyworm which overwinter in Ontario but attack earlier in the spring).  Fall armyworm are a very sporadic pest, but can cause extensive damage if you don’t catch it on time.  The moths lay eggs in corn (especially fields with grassy weed escapes) and cereal crops.  The larva feed voraciously on corn leaves and, though rare for them to so, they are capable of completely stripping plants of leaves. The larva vary in base colour from light tan to green to almost black; all of them will have 3 yellow-white lines down their back with darker lines on either side of the white line.   Near the bottom on each side (below the other lines) there is a wavy yellow line with red blotches through it.  The larva also have a dark black head with an upside down white Y shape on the front of it. 

Fall Armyworm larva

 Control is warranted if you have 4 or more unparasitized larva per square foot AND they are <1” long.  Once the larva are >1” long they become more resistant to insecticides (due to bigger size requiring them to be exposed to a higher dose to kill them).  Parasitoids can lay eggs on the armyworm, and the larva that hatch from these eggs will burrow into the armyworm, feed on them and kill them.  The most obvious sign of these parasitoids is the “cocoon” they make on the outside of the armyworm to pupate in.  Armyworm with 1 or more of these cocoons on them shouldn’t be counted when determining if control is required.

Parasitized armyworm


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