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Species of moth

The cabbage looper (Trichoplusia ni) is a medium sized moth in the family Noctuidae, a family commonly referred to as owlet moths. Its common name comes from its preferred host plants and distinctive crawling behavior. Cruciferous vegetables, such as cabbage, bok choy, and broccoli, are its main host plant; hence, the reference to cabbage in its common name. The larva is called a looper because it arches its back into a loop when it crawls. The adult cabbage looper is a migratory moth that can be found across North America and Eurasia, as far south as Florida and as far north as British Columbia. Its migratory behavior and wide range of host plants contribute to its broad distribution. The cabbage looper larva is a minor vegetable pest, especially for crucifers. While it is not significantly destructive, it is becoming difficult to manage due to its broad distribution and resistance to many insecticides. Numerous methods are being researched in order to control this species.

The cabbage looper larva is a type of cabbage worm, a general term for a Lepidopteran pest that primarily feeds on crucifers. They closely resemble each other, in that they are all smooth and green, but they are not closely related in terms of phylogeny. In fact, none of the cabbage worms bear close phylogenetic relations, as they are all from different families. It is related to other vegetable pests, like the cutworm and armyworms.

Conventional mate finding strategy involves males seeking and competing for females and females caring for offspring. In many animals, however, the opposite occurs, where the females competes for males and males care for young. This role reversal can occur for a variety of reasons: environmental conditions, timing of fertilization, and biased sex ratios. For example, male fish often provide more parental care because, after females lay their eggs, males have to ensure that their sperm fertilizes the eggs and does not get washed away. It may be beneficial for the female to lay more eggs instead of caring for the eggs, so she departs as the male fertilizes the eggs, leaving him to care for the eggs. The cabbage looper generally utilizes typical mating strategies, in that males compete for females. However, occasionally the reverse occurs, where females will seek males. This only happens under particular selection conditions, such as a shortage of males or host plants that bias the sex ratio towards females.

The cabbage looper eggs are generally yellow white in color, dome shaped, and patterned with ridges. They are 0.6mm in diameter and 0.4mm in height, and they are usually laid singly on the underside of leaves. In one day, 40–50 females can lay 1000–2000 viable eggs. Viable eggs hatch after about three days, while unviable eggs fail to develop and collapse within that period. Eggs are mostly found on leaves that are both larger and higher on the plant. It is not clear why eggs are preferentially laid on these leaves.

Cabbage looper larvae are a type of cabbage worm, green in colour with a white stripe on the side. After hatching, they are green and slightly hairy, but eventually turn green and lose the hair, leaving only a few bristles. They are identified by their looping behaviour, in which they arch their body in a loop when they crawl. Larvae are generally 3–4  cm long, and can have four to seven instars within 9–14 days. Mating occurs 3 or 4 days after metamorphosis, during which 300–1400 eggs are oviposited. Cabbage looper populations in North America migrate from Mexico to Canada, depending on the seasons. It generally overwinters in Mexico or southern California, where temperatures are above even during winter. It used to be frequently found in Florida, but this has lessened due to fewer cabbage crops. During summer, it is less commonly found in southern regions, due to high temperatures. Similar to the monarch butterfly, populations presumably migrate in groups, as there is little genetic difference between source and migrating populations. Similar seasonal distributions were found in Europe. There, the cabbage looper can be found from England to southeastern Europe. Temperature does not affect the pheromone sensitive receptor neurons.

The cabbage looper is a generalist insect that can reside and feed on over 160 host plants. The looper's variety of hosts is partially due to the ability of its salivary glands to differentially express based on the host. For example, cabbage and tomato plants use defensive strategies involving different compounds, and the cabbage looper can combat either by upregulating the appropriate genes. The gland's high responsiveness to the diet allows for considerable flexibility in host plants. The cabbage looper's preferred hosts are crucifers such as cabbage and broccoli, because it grows faster on these plants, possibly due to nutritional or chemical differences. Tobacco can also be a host for the cabbage looper. However, it is not preferred because gummosis, a gummy substance produced by some plants, and trichomes, hair like appendages, harm early larvae survival. Older larvae are more resistant to these defenses. The number of caterpillars on a plant can depend on a plant's maturity. Cabbages that mature early are less attractive, whereas cabbages just beginning to head are the most attractive. Among crucifers, there generally seems to be no preference for one specific type of crucifer, like kale over cabbage or broccoli over brussels sprouts. The only apparent preference is for red cabbage – nearby double the number of caterpillars were present on the red cabbage compared to the green. This suggests that the number of caterpillars on a host plant has less to do with the species of host than with the host's height and foliage.

Similar to other pheromone biosynthesis reactions, female cabbage looper pheromone production initiates with synthesis of 16 and 18 carbon fatty acids. This is followed by desaturation at C1 and chain shortening by two or four carbons. Finally, the fatty acid is reduced and acetylated to form an acetate ester. The result is a blend of different female pheromone compounds at a consistent ratio. This ratio can be highly altered by mutations in chain shortening proteins, demonstrating that the chain shortening step is important for determining the ratio of pheromones in the final blend. As a species, the cabbage looper does not hormonally regulate pheromone production. Stage specific proteins correspond to the development of the pheromone gland. The immature gland lacks numerous enzymes crucial to pheromone biosynthesis, such as fatty acid synthetase and acetyltransferase, which is why the looper cannot produce pheromones prior to the adult stage. Upon complete development of the pheromone glands at the adult stage, pheromones are constantly produced.

Cabbage loopers possess olfactory receptor neurons on their antennae for detecting pheromones. The neurons are specifically located on two sensory structures called sensilla that differ in length and pore density. Male loopers have two types of neurons, and depending on which sensilla that are present, the neurons will detect female pheromones at varying sensitivities to each of the six pheromones. The neurons are most sensitive to the main component of the female pheromone blend, cis 7 dodecenyl acetate, and the male inhibitory signal, cis 7 dodecenol. The presence of cis 7 dodecenyl acetate is crucial for male response to female pheromones, as it is 80% of the entire blend. The base region of the antennae, where receptor neurons for this pheromone are located, has more sensory structures than the ends. The base region is also less likely to experience damage, showing the importance of detecting the pheromone. It is not clear why male neurons detect the inhibitory compound, as there is no evidence showing that females produce this compound. One possibility is that its presence in the female pheromone blend may be too small to be detected by scientific equipment. The inhibitory signal only elicits a response when delivered alongside female pheromones to avoid mixing signals from other species, suggesting that while it cannot be detected in the female pheromone blend, it has an important role in female detection. These neurons are also capable of recognizing and responding to cis 7 tetradecenyl acetate and cis 9 tetradecenyl acetate. There are no specialized neurons for the other three pheromones.

General predators like spiders, ants, and lady beetles prey on cabbage looper eggs and larvae, removing 50% of the eggs and 25% of the larvae within three days. Lady beetles consume at the highest rate. Other common predators of cabbage looper larva include Orius tristicolor, Nabis americoferus, and Geocoris pallens.

While the cabbage looper frequently encounters parasites, its most common parasite is the tachinid fly. In one study, 90% of the parasitized larvae were due to the tachinid fly. It parasitizes most often in the late fall and winter, but it is capable of parasitizing year round. Cabbage loopers at their third or fourth instar yield the most parasites. It is early enough in the larval stage that the maggots still have time to feed and grow before pupation can prevent parasite emergence. It is also late enough that the caterpillars are large enough to support the maggots. Fly oviposition is often triggered by the larva thrashing to repel the fly, regardless of whether the larvae are already parasitized. As a result, larvae are often overparasitized, overwhelming and killing smaller larvae. During oviposition, the mother glues the fly egg to the host. This helps the maggot burrow into the larva, where it remains until the third day. The maggot cuts a slit into the back and eats its way out of the larva.

The moth is susceptible to viral diseases including nucleopolyhedrovirus (NPV). This is a naturally occurring virus whose natural hosts include Lepidoptera, arthropods, and Hymenoptera. From the family Baculoviridae, it is a type of Alphabaculovirus and its genome is 80–180kb long. NPVs are commonly used as pesticides for the cabbage looper. There are numerous NPVs, many of which were isolated from the cabbage looper or the alfalfa looper. NPVs vary in infectivity and virulence. For example, the AcMNPV isolates are more infectious than the TnSNPV (the SNPV/single nuclear polyhedrosis virus specific to the cabbage looper) isolates in the first instar, while the TnSNPV isolates produced more occlusion bodies, protein structures that protect the virus and increase long term infectivity. TnSNPVs are their most lethal during the third and fourth instars; they have detrimental effects such as delayed development, reduced egg production, and fewer hatched eggs. These effects are significantly diminished when the larvae are infected during the fifth instar, suggesting that the earlier infection is more effective. Bacillus thuringiensis (Bt) is a gram positive soil bacterium from the phylum Bacillota. It is often used as a biological insecticide for numerous insect pests, including the cabbage looper, and reduces both growth rate and pupal weight. The cabbage looper has demonstrated resistance to Bt, specifically the toxin Cry1Ac, due to an autosomal recessive allele. Although it is not entirely clear which gene causes the resistance phenotype, there is strong evidence supporting the correlation between a mutation in the membrane transporter ABCC2 and Bt resistance. Other studies with greenhouse evolved population of Bt resistant cabbage looper demonstrate that the downregulation of the aminopeptidase N, APN1, results in its resistance.

The cabbage looper genome is 368.2 Mb long (scaffold N50=14.2 Mb; contig N50=621.9 kb; GC content=35.6%) and includes 14,037 protein coding genes and 270 microRNA (miRNA) genes. The genome and annotation are available at the Cabbage Looper Database. The cabbage looper genome is larger than the Drosophila melanogaster genome (180Mb) but smaller than the Bombyx mori genome (530mb). It encodes at least 108 cytochrome P450 enzymes, 34 glutathione S transferases, 87 carboxylesterases, and 54 ATP binding cassette transporters, some of which may be involved in its insecticide resistance. The PiggyBac Transposon, a widely used tool for genetic engineering, was originally discovered in the cabbage looper and subsequently identified in other taxa as well.

Similar to the diamondback moth, the cabbage looper is one of the most problematic cabbage pests. The larvae eat large holes in the underside of leaves and consume developing cabbage heads. In addition, they leave behind sticky frass, contaminating the plants. They also consume the leaves of myriad host plants beyond cabbages. Although it is a damaging pest, the cabbage looper can be tolerated. For example, plant seedlings can endure the cabbage looper. However, the cabbage looper becomes more problematic once the plant begins heading.

There is extensive research in cabbage looper pheromones for the goal of developing traps to catch the moth. Initial research involved isolation of the female pheromone to identify the compounds and potentially synthetically replicate the natural female pheromone. Scientists were able to develop a synthetic version that functions biologically like the natural form. The synthetic female pheromone has been used with black light traps to study cabbage looper populations in various regions of the US.

Scientists are actively seeking methods for controlling the cabbage looper. Known as an evolutionary arms race, scientists are constantly researching ways to control the cabbage looper while the looper evolves resistance to the management methods. Synthetic insecticides are relatively effective; however, many of them are banned for their toxicity. One exception is Ambush. Studies have shown that this pyrethroid insecticide is effective at killing cabbage looper eggs, and its usage is permitted in the US. Other studies have explored the usage of biological insecticides; for example, a polyhedrosis virus was shown to be effective. Unfortunately, managing large quantities of this virus would be difficult, so it is not a feasible option. An effective option is to use synthetic and biological insecticides together; this method seems to both control the population and slow the development of resistance, but it still requires the usage of toxic chemicals. Currently, spraying Bacillus thuringiensis is considered to be the best option, possibly with NPV for an added benefit, but cabbage looper is growing increasingly more resistant to B. thuringiensis. Recent studies, however, have demonstrated that cabbage loopers resistant to B. thuringiensis are twice as susceptible to NPVs, which provides insight into novel biological control methods.

Baculovirus insect cell expression is a technique used to produce large quantities of a desired protein. It takes advantage of the ability of Baculovirus to insert genes into its target cell and induce protein expression this gene. Numerous insect cells have been developed into cell lines, such as fruit flies, mosquitoes, and silkworms. The tissue of the cabbage looper has also been used to develop a cell line. It is particularly useful for its fast growth rate and less reliance upon insect haemolymph in the medium. The cabbage looper cell line has also been engineered to grow in serum free media. Although serum helps insect cell growth, it is very expensive and can hinder subsequent experimental procedures. As a result, the development of the cell line to grow independently of serum means that the cell line could be used to produce viruses and proteins in a more affordable, efficient, and productive manner.