The cabbage looper completes its entire life cycle, from egg to adult moth, in 18 to 25 days depending on temperature. Warmer conditions speed development, while cooler weather stretches each stage out considerably. Understanding each phase helps you identify these pests early and time any intervention for when it matters most.
Egg Stage: 2 to 10 Days
Female moths lay their eggs singly on the undersides of host plant leaves. The tiny, dome-shaped eggs are pale green to white and easy to overlook. Hatching time depends almost entirely on temperature: at warm summer temperatures around 90°F (32°C), eggs hatch in about two days. At a moderate 80°F (27°C), expect three days. Cool spring weather around 68°F (20°C) stretches incubation to five days, and near 60°F (15°C) the eggs can take almost 10 days to hatch.
A single female produces 300 to 600 eggs over her lifetime, depositing them across multiple plants. This high output is one reason cabbage looper populations can build quickly in a garden or field, especially during warm stretches when generations overlap.
Larval Stage: 18 to 21 Days
The larval stage is the longest and most destructive phase. Once hatched, the caterpillar passes through five instars (growth stages), though some individuals go through as few as four or as many as seven. Total larval development takes roughly 18 to 21 days across typical summer temperatures. The caterpillars are pale green with narrow white lines running down each side, reaching about 1.5 inches long when fully grown.
The name “looper” comes from how these caterpillars move. They lack legs in their midsection, so they arch their bodies into a loop with each step, pulling their back end forward to meet the front. This distinctive inchworm motion is the easiest way to tell a cabbage looper apart from other green caterpillars on your plants.
Early instars cause minimal damage, feeding on the upper leaf surface near the midrib. As the larvae grow, they chew large, irregular holes between the major leaf veins. Severe feeding can stunt cabbage and cauliflower heads, and damage to developing buds on young cabbages can cause heads to abort entirely. Older larvae also bore into heads, and the large quantities of greenish-brown droppings they leave behind contaminate foliage even where direct feeding damage is minor.
Young plants between the seedling stage and heading can actually tolerate substantial leaf loss without affecting yield. The real economic and garden damage comes after heading begins, when larvae bore into the parts you harvest.
Pupal Stage: 4 to 13 Days
When the caterpillar finishes feeding, it spins a thin, gauzy cocoon directly on the plant rather than burrowing into the soil like many other moth species. The cocoon is flimsy and nearly transparent, so you can sometimes spot the green-to-brown pupa through the silk if you look carefully at the underside of leaves. Pupation lasts about four days at 90°F, six days at 80°F, and 13 days at 68°F.
Adult Moth Stage: 10 to 12 Days
The adult cabbage looper moth lives an average of 10 to 12 days. It has a wingspan of about 1.3 to 1.5 inches, with mottled gray-brown forewings and lighter hind wings that darken toward the edges. The most reliable identifying feature is a pair of silvery-white marks near the center of each forewing: a U-shaped spot and a small circle or dot, often connected. These markings distinguish it from most other moths you’ll find around vegetable crops.
Moths are most active at night but sometimes feed and lay eggs around dusk, earning them the label “seminocturnal.” They can also become active on overcast days. Females will lay eggs at temperatures as low as about 60°F, though flight activity picks up on warmer evenings. Because of their nighttime habits, you rarely see the adults unless you’re checking plants after dark or using pheromone traps.
Generations Per Season
With an egg-to-adult cycle as short as 18 days in hot weather, cabbage loopers can produce multiple overlapping generations in a single growing season. In warm southern climates, this can mean continuous pressure on crops from spring through fall. In cooler northern regions, two to three generations per year are more typical. Cabbage loopers do not survive freezing winters in most of the United States; instead, moths migrate northward each spring from southern overwintering areas.
Host Plants and Feeding Damage
All cole crops are primary targets: cabbage, broccoli, Brussels sprouts, cauliflower, and rutabaga. But cabbage loopers are not picky. They also feed on beets, celery, lettuce, peas, potatoes, spinach, and tomatoes. Damage to root crops is generally minor since the larvae focus on foliage, but leafy crops and heading vegetables take the worst hit.
Telling Loopers Apart From Similar Caterpillars
Three green caterpillars commonly show up on cole crops, and they’re easy to confuse at a glance.
- Cabbage loopers are pale green with white side stripes, grow to 1.5 inches, and move with a distinctive looping motion because they have no legs in the middle of their body.
- Imported cabbageworms are velvety green with faint yellow stripes, grow to about 1 inch, and move sluggishly when disturbed. They have a full set of prolegs and crawl normally.
- Diamondback moth larvae are light green, tapered at both ends, and much smaller at only about one-third of an inch long. They wiggle vigorously when touched.
The looping crawl is the fastest field test. If the caterpillar arches its body into a hump as it moves, it’s a cabbage looper.
Timing Control to the Life Cycle
The larval stage is the only window where cabbage loopers cause damage, and younger larvae are far easier to manage than older ones. Biological insecticides based on Bt (a naturally occurring soil bacterium) work best against small, early-instar caterpillars because the toxin must be eaten to be effective, and younger larvae consume it more readily relative to their body size. By the time larvae reach their final instars, they’re large, harder to kill, and have already done most of their damage.
Checking the undersides of leaves weekly for eggs and small caterpillars lets you act before feeding becomes severe. Row covers placed over transplants at planting time physically block moths from laying eggs and can prevent infestations entirely if sealed at the edges.