Cole crop caterpillars make a holey mess

Marissa Schuh,IPM extension educator.

Brassicas are being harvested, and some growers are finding lots of holes in their cabbage, collards, and kale. In Minnesota we have three species of caterpillars that feed on the foliage of all our brassica crops. Read on to get to know these caterpillars and when it is worth managing them.

Size alone isn’t enough for caterpillar ID. Diamondback moth is smooth, and tapered at each end. Imported cabbageworm is velvety. Cabbage looper and smooth and moves in an inchworm fashion. Photo: Marissa Schuh, University of Minnesota Extension.

The three cole crop caterpillars we see in Minnesota are present from transplanting in May to final harvest in the fall.  The varying biology and multiple generations mean there is probably always some small green caterpillar feeding in your cole crops.  To manage them well, you need to be able to tell who is who.

The first on the scene- Diamondback moth

• Diamondback moth is the first arrival most years, with larvae chewing many small holes in leaves. They sometimes are unable to chew through the whole leaf, leaving a thin, clear leaf layer behind.  
• Adults are unlikely to be noticed, as they are nondescript moths (small, brown). 
• Caterpillars are small (max size is 0.45 inches) and green, with tapered edges. If you poke at them, diamondback moth caterpillars will wiggle wildly and roll off the leaf, sometimes hanging by a silk thread.

Diamondback moth. Image: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The fuzzy menace -- Imported Cabbageworm

• Imported cabbageworm shows up at some point in May.
• The adults a white butterflies, commonly seen fluttering around fields and field edges.
• Caterpillars can be up to an inch and size, though are smaller when they are young. They have a velvet fuzz covering their green bodies. If you look closely, they have yellow stripes running down their sides.
• These guys care less when bothered, they tend to react sluggishly.

Various sizes of imported cabbageworm. Photo: Ansel Oommen, Bugwood.org

The migratory muncher -- Cabbage looper

• Cabbage looper is the last arrival. Unlike the other two, cabbage loopers cannot survive the Minnesota winters, so when they show up in the season is dependent on weather fronts bringing them up from the southern US.
• Adult are larger, but still generic, brown moths.
• Caterpillars can be up to an inch and a half long. They are smooth, dusty green with white racing stripes running down the length of their bodies. When these caterpillars move, they arch their bodies like an inchworm.

Cabbage looper showing off  its stripes and inchworm walk. Photo:  Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Why does ID matter? Because each species can do different amount of damage.

Scouting Thresholds for Caterpillar Pest in Broccoli and Cauliflower
From Michigan State University’s Caterpillar Pests in Cole Crops.

Plants in Seedbed Stage

• Imported Cabbageworm, Cabbage Looper - 10% of plants with larva
• Diamondback moth- 10% of plants with larvae

Plants in Transplant through First Curd Stage

• Imported Cabbageworm, Cabbage Looper - 40% of plants with larvae
• Diamondback moth- 20% of plants with larvae

Plant in Fist Curd to Final Harvest

• Imported Cabbageworm, Cabbage Looper - 10% of plants with larvae
• Diamondback moth- 10% of plants with larvae

Scouting Thresholds for Caterpillar Pests in Cabbage
From University of Minnesota

Plants in Transplant through First Cupping Stage

• Imported Cabbageworm, Cabbage Looper - 10% of plants with larvae
• Diamondback moth- 10% of plants with larvae

Plants in Transplant through First Curd Stage

• Imported Cabbageworm, Cabbage Looper - 30% plants infested with 1 or more medium-large imported cabbageworm larvae and/or 1 or more cabbage looper eggs or larvae.
• 50% plants infested with 5 or more larvae each.

Plant in Cupping to Harvest Stage

• Imported Cabbageworm, Cabbage Looper - 10% plants infested with 1 or more medium-large imported cabbage worm larvae and cabbage looper eggs or larvae.
• Diamondback moth- 10% plants infested with 1 or more larvae each.

Management options
Bt is effective against all caterpillars. Understanding the nuances of using Bt is key in its successful control. 

• Caterpillars must ingest the product, this means it needs to reach the caterpillars where they are feeding. The product may need to be on the underside of the leaves, or the tightly curled leaves of the new growth.
• Bt breaks down in direct sunlight and washes away with water, so reapplication will be needed.
• Bt works best against these caterpillars when they are young and small (under a quarter inch in size). This highlights how scouting and early detection will help catch these pests when they are most easily controlled.

Spinosad is another organic option that performs well.  Conventional growers have many options, see the Midwest Vegetable Production Guide for options. For more pictures and identification tips, see Cole Crop Caterpillars.