Cabbage looper facts for kids

Kids Encyclopedia Facts

Quick facts for kids Cabbage looper

Scientific classification
Synonyms
Phytometra brassicae Plusia innata Herrich-Schaffer, 1868

The cabbage looper (Trichoplusia ni) is a medium-sized moth. It belongs to the family Noctuidae, also known as owlet moths. Its name comes from the plants it likes to eat and how it moves.

It mainly eats cruciferous vegetables like cabbage, bok choy, and broccoli. This is why "cabbage" is in its name. The young form, called a larva, is called a "looper" because it arches its back into a loop when it crawls.

Cabbage loopers prefer cruciferous plants. However, they can eat over 160 different types of plants. Adult cabbage loopers travel long distances. They can be found across North America and Eurasia. They live as far south as Florida and as far north as British Columbia. Their travels and wide range of food plants help them spread widely.

The cabbage looper larva is a small pest for vegetables. It especially affects cruciferous plants. It does not cause huge damage. But it is becoming hard to control. This is because it is found everywhere and resists many bug sprays. Scientists are looking for new ways to manage this species.

What is a Cabbage Looper?

The cabbage looper larva is a type of cabbage worm. This is a general name for insect pests that eat mainly cruciferous plants. Cabbage loopers look very similar to other cabbage worms. They are all smooth and green. But they are not closely related in their family tree.

In fact, most cabbage worms are from different insect families. The cabbage looper is part of the Noctuidae family. This is one of the largest families of Lepidoptera (moths and butterflies). It is related to other plant pests. These include the cutworm and armyworms.

Reproduction and Life Cycle

How Cabbage Loopers Mate

When ready to mate, cabbage loopers show off. They lift their abdomen and fan their wings. Male moths also fan out hairs on their abdomen. They open their claspers and show part of their spermatophores. Males show more of their spermatophores as they wait.

If interested, a potential mate touches the other's abdomen with its antennae. Mating happens if both agree. Mating usually occurs around 2 AM. It can happen between midnight and 4 AM. Moths generally mate 3 to 4 days after they become adults. They can mate up to 16 days later. Eggs are not fully ready when the moth first appears. They need a few days to mature.

Cabbage loopers often mate with more than one partner. This is called multiple matings. This strategy can help both males and females. For female cabbage loopers, laying more eggs happens when they mate more. This means they lay more eggs in total. It was once thought that many matings were needed to fertilize all eggs. But now we know one mating is enough.

Instead, the male's spermatophore likely gives nutrients to the female. These nutrients help her lay more eggs. This might be why males make special scents to attract females. Females might be looking for these nutrient-rich spermatophores. For male cabbage loopers, mating many times does not harm their spermatophores. This means they can have many chances to reproduce.

Laying Eggs (Oviposition)

After mating, the female moth looks for a host plant. She then lays her eggs. This is called oviposition. Females can even lay eggs without mating. This can happen soon after they become adults. However, these eggs are not fertile. Eggs do not become fertile until the third day of adulthood.

The female chooses a host plant based on what the larvae learned. This is called learned host behavior. Moths that do not know a plant will avoid laying eggs on it. They prefer to lay eggs on a plant they know. This happens even if the familiar plant has bad-tasting chemicals. This shows that larvae and moths learn their plant choices. They are slow to realize if a plant chemical is harmful.

Insect waste, called larval frass, also affects the choice. Its presence tells mothers that the spot is already taken. This helps avoid too many larvae in one place.

Life Cycle Stages

Egg Stage

Cabbage looper eggs are usually yellow-white. They are dome-shaped and have ridges. They are about 0.6 mm wide and 0.4 mm tall. Females usually lay them one by one on the underside of leaves. In one day, 40-50 females can lay 1000-2000 good eggs.

Good eggs hatch after about three days. Eggs that are not good fail to grow and collapse. Eggs are mostly found on larger leaves that are higher on the plant. Scientists are not sure why eggs are laid on these specific leaves.

Larva

Larva Stage

Cabbage looper larvae are a type of cabbage worm. They are green with a white stripe on their side. When they first hatch, they are green and a bit hairy. Later, they become smooth green with only a few bristles. You can spot them by their looping movement. They arch their body into a loop when they crawl.

Larvae are usually 3–4 cm long. They can go through four to seven growth stages, called instars. This takes 9–14 days. Larvae do not eat much at first. But they eat more and more as they grow. By the end, they eat three times their own weight daily.

Pupa Stage

Pupa

When they pupate, they attach to the underside of leaves. They form a silky cocoon. This stage can last 4–13 days. It depends on the temperature around them. Male pupae are a little bigger than female pupae.

Adult Stage

The adult is a moth. It has gray-brown front wings and light brown back wings. It is about 2.5 cm long. Its wingspan is about 3.8 cm. Adult moths are active at night. They spend their days hidden by their host plants. They start moving about 30 minutes before sunset.

Males have light brown hairs that lie flat on their abdomen. This helps tell them apart from females. Mating happens 3 or 4 days after they change into adults. Females then lay 300-1400 eggs. The entire life cycle, from egg to adult, usually takes 24–33 days.

Where Cabbage Loopers Live and Travel

The cabbage looper lives across North America and Eurasia. It can be found as far south as Florida and as far north as British Columbia.

In North America, cabbage looper groups travel from Mexico to Canada. This depends on the seasons. They usually spend winter in Mexico or southern California. Temperatures there stay above 16°C (61°F) even in winter. They used to be common in Florida. But this has changed due to fewer cabbage farms.

As northern parts of North America get warmer, cabbage loopers move north. They only travel if the area is above 16°C. In summer, they are less common in southern areas. This is because of the high temperatures. Like the monarch butterfly, these groups likely travel together. This is because there is little genetic difference between groups.

Similar seasonal patterns are seen in Europe. There, the cabbage looper can be found from England to southeastern Europe.

How Temperature Affects Cabbage Loopers

Cabbage looper travel patterns depend a lot on temperature. Temperature can affect how they grow. It has the biggest impact on the pupation stage. Pupae often stop changing into adults if they grow at 10°C (50°F). Even if moved to warmer temperatures, they often come out deformed.

Temperatures above 35°C (95°F) also cause problems for adults. Their wings might not develop well. Mating and flying are affected by temperatures above 32°C (89.6°F) and below 16°C (61°F). This might explain why they move to northern areas when it gets to 16°C.

The time it takes for a male to respond to a female increases with temperature. But when it reaches 27°C (80.6°F), mating actually increases. At the same time, egg-laying and lifespan decrease. Hatching almost stops at 32°C (90°F). The egg itself is quite tough. It can develop at 10°C and 40°C (104°F). However, even if developed, it might not hatch. Temperature does not affect the special cells that sense scents.

What Plants Cabbage Loopers Eat

The cabbage looper is a generalist insect. This means it can live and feed on over 160 different host plants. The looper's wide range of food comes partly from its salivary glands. These glands can change based on the plant it eats. For example, cabbage and tomato plants use different defenses. The cabbage looper can fight both by turning on the right genes.

The gland's quick response to food allows it to eat many plants. The cabbage looper prefers crucifers like cabbage and broccoli. It grows faster on these plants. This might be due to nutrients or chemicals in the plants. Tobacco can also be a host plant. But it is not preferred. This is because of sticky substances and hair-like parts on tobacco plants. These can harm young larvae. Older larvae are stronger against these defenses.

The number of caterpillars on a plant can depend on how mature the plant is. Cabbages that grow early are less appealing. Cabbages that are just starting to form a head are the most appealing. Among crucifers, there seems to be no strong preference for one type. For example, they do not prefer kale over cabbage or broccoli over Brussels sprouts. The only clear preference is for red cabbage. About twice as many caterpillars were found on red cabbage compared to green. This suggests that the number of caterpillars depends more on the plant's height and leaves than its specific type.

How Cabbage Loopers Find Food by Smell

Cabbage loopers use plant smells to find food. They also use smells to find good plants for laying eggs. This helps them survive and reproduce. Mated female moths respond faster to plant smells. This is compared to unmated females and males. This difference might be because mated females need plants for both food and egg-laying. Unmated moths mostly need plants for food. So, mated females are more driven to find a host plant.

The cabbage looper is attracted to these plant smells:

phenylacetaldehyde
methyl salicylate
2-phenylethanol
benzaldehyde
benzyl alcohol
benzyl acetate
methyl-2-methoxy benzoate

The strongest attractor is phenylacetaldehyde. But the cabbage looper is more attracted to a mix of smells than to phenylacetaldehyde alone.

Pheromones

Male Pheromones

d-linalool
m-cresol
p-cresol

Male cabbage loopers also make special scents called pheromones. These come from hair-like structures on their abdomen. Different mixes of pheromones help males attract mates. Some pheromone parts are more appealing to females. Cresol is important for attracting females. Linalool is found in flower smells. It is thought to attract moths looking for food.

Males near host plants are more attractive to females. This is because plant smell makes the male pheromone more appealing. This helps females choose a mate. Males whose pheromones are enhanced by plant smell are more likely to be near a host plant. Male pheromones might also help with finding food. Both males and females are more attracted to male pheromones when they are hungry.

Female Pheromones

cis-7-dodecenyl acetate
cis-5-dodecenyl acetate
11-dodecenyl acetate
cis-7-tetradecenyl acetate
cis-9-tetradecenyl acetate
dodecyl acetate

Cabbage loopers are special because both females and males release pheromones to find a mate. Usually, females release pheromones from their abdomen tips. Males then find the females when they smell these pheromones. Females near host plants are more attractive to males. This might be because females release more pheromones when host plant smell is around.

It is not clear why host plant smells make females produce more pheromones. But this response might help them find a mate faster. This increases their chances of mating. Female cabbage loopers usually attract the male. This is because females have more to lose by spending energy and time looking for a mate.

How Pheromones Are Detected

Cabbage loopers have special cells on their antennae. These are called olfactory receptor neurons. They help detect pheromones. These cells are on two sensory structures called sensilla. These sensilla differ in length and how many pores they have.

Male loopers have two types of neurons. Depending on which sensilla are present, these neurons detect female pheromones. They have different sensitivities to each of the six pheromones. The neurons are most sensitive to the main part of the female pheromone mix. This is cis-7-dodecenyl acetate. They also detect a male signal that stops other males, cis-7-dodecenol.

The presence of cis-7-dodecenyl acetate is very important for males to respond. It makes up 80% of the whole mix. The base of the antennae has more sensory structures. This is where the neurons for this pheromone are located. This area is also less likely to get damaged. This shows how important it is to detect this pheromone.

It is not clear why male neurons detect the stopping compound. There is no proof that females produce this compound. One idea is that it is present in the female pheromone mix. But it might be too small to be found by scientific tools. The stopping signal only works when given with female pheromones. This helps avoid mixing signals from other species. This suggests it has an important role in female detection.

These neurons can also recognize cis-7-tetradecenyl acetate and cis-9-tetradecenyl acetate. There are no special neurons for the other three pheromones. Instead, these smaller pheromones can activate neurons that are not specialized. This is why mixes that are missing one or two minor pheromones can still fully activate the male receptors.

Enemies of the Cabbage Looper

Predators

Common predators like spiders, ants, and lady beetles eat cabbage looper eggs and larvae. They can remove 50% of the eggs and 25% of the larvae in three days. Lady beetles eat the most. Other common predators include Orius tristicolor, Nabis americoferus, and Geocoris pallens.

Parasites

The cabbage looper often encounters parasites. Its most common parasite is the tachinid fly. In one study, 90% of infected larvae were due to this fly. It infects most often in late fall and winter. But it can infect year-round. Cabbage loopers at their third or fourth growth stage have the most parasites. This stage is early enough for the maggots to feed and grow. It is also late enough that the caterpillars are big enough to support the maggots.

The fly lays its eggs when the larva thrashes to get rid of the fly. This happens even if the larvae are already infected. So, larvae are often over-infected. This can overwhelm and kill smaller larvae. When laying eggs, the mother fly glues the egg to the host. This helps the maggot burrow into the larva. It stays there until the third day. The maggot then cuts a slit in the larva's back and eats its way out.

Diseases

The moth can get viral diseases. These include nucleopolyhedrovirus (NPV). This is a natural virus. It infects Lepidoptera, arthropods, and Hymenoptera. It is a type of Alphabaculovirus from the Baculoviridae family. Its genetic material is 80-180kb long. NPVs are often used as pesticides for the cabbage looper.

There are many NPVs. Many were found in the cabbage looper or the alfalfa looper. NPVs differ in how easily they infect and how deadly they are. For example, AcMNPV types are more infectious than TnSNPV types in the first larval stage. But TnSNPV types produce more occlusion bodies. These are protein structures that protect the virus and help it infect for longer. TnSNPVs are most deadly during the third and fourth larval stages. They cause problems like slow growth, fewer eggs, and fewer hatched eggs. These effects are much smaller if larvae are infected in the fifth stage. This suggests that earlier infection works better.

Bacillus thuringiensis (Bt) is a type of bacteria found in soil. It is often used as a natural bug killer for many insect pests. This includes the cabbage looper. It slows growth and reduces pupal weight. The cabbage looper has shown resistance to Bt. Specifically, it resists the toxin Cry1Ac. This is due to a genetic change.

Scientists are not entirely sure which gene causes this resistance. But there is strong evidence. A change in the ABCC2 gene is linked to Bt resistance. Other studies show that a decrease in the APN1 protein leads to resistance. This was seen in cabbage loopers that developed resistance in greenhouses.

Cabbage Looper and Humans

Damage to Crops

Like the diamondback moth, the cabbage looper is a big problem for cabbage farms. The larvae eat large holes in the underside of leaves. They also eat developing cabbage heads. Plus, they leave behind sticky waste, called frass. This makes the plants dirty. They also eat the leaves of many other host plants.

Even though it is a damaging pest, some damage can be tolerated. For example, young plant seedlings can handle the cabbage looper. However, the cabbage looper becomes a bigger problem once the plant starts forming a head. This pest is known for easily infecting many crops. It is also getting harder to control. This is because the cabbage looper is becoming resistant to both natural and man-made bug sprays.

How to Manage Cabbage Loopers

Pheromone Traps

Scientists have done a lot of research on cabbage looper pheromones. The goal is to make traps to catch the moths. Early research involved finding the female pheromone. They wanted to identify the chemicals and possibly make a copy. Scientists were able to create a man-made version. It works just like the natural female pheromone.

This man-made female pheromone has been used with black light traps. These traps help study cabbage looper groups in different parts of the US. Man-made male pheromones have also been made. They were found to attract and trap both male and female cabbage loopers. The mix of male pheromones helped trap females looking for mates. It also trapped moths looking for food.

Later studies in Arizona showed that pheromone traps with black lights are not enough to control the cabbage looper. The traps did catch some males. This led to less mating and fewer eggs laid. But the effect was not big enough to stop using bug sprays. This is because farming rules often require crops to be almost completely bug-free.

Insecticides (Bug Sprays)

Scientists are actively looking for ways to control the cabbage looper. This is like an "evolutionary arms race." Scientists keep researching ways to control the looper. But the looper keeps developing resistance to these methods. Man-made bug sprays are quite effective. However, many are banned because they are toxic.

One exception is Ambush. Studies show that this pyrethroid bug spray kills cabbage looper eggs. Its use is allowed in the US. Other studies have looked at using natural bug sprays. For example, a polyhedrosis virus was shown to be effective. But managing large amounts of this virus would be hard. So, it is not a practical option.

Using man-made and natural bug sprays together can be effective. This method seems to control the population. It also slows down the development of resistance. But it still requires using toxic chemicals. Currently, spraying Bacillus thuringiensis (Bt) is thought to be the best option. Adding NPV might also help. But cabbage looper is becoming more resistant to Bt.

Recent studies show something interesting. Cabbage loopers resistant to Bt are twice as likely to be affected by NPVs. This gives new ideas for natural control methods.

Use in Research

Baculovirus-insect cell expression is a method used to make large amounts of a specific protein. It uses the ability of Baculovirus to put genes into a target cell. This makes the cell produce the desired protein. Many insect cells have been grown in labs. These include cells from fruit flies, mosquitoes, and silkworms.

Tissue from the cabbage looper has also been used to create a cell line. It is especially useful because it grows fast. It also needs less insect fluid in its growth medium. The cabbage looper cell line has also been changed to grow without serum. Serum helps insect cells grow. But it is very expensive. It can also make later experiments harder. So, developing a cell line that grows without serum means it can be used to make viruses and proteins more cheaply, efficiently, and productively.