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Helicoverpa assulta or H. assulta is a moth found in Asia, Africa, and Australia. It is a pest of economically viable crops including tomatoes, tobacco, and hot peppers. Because of its status as a pest, understanding how to control the moth’s behavior is a priority, but the species’ larval eating behavior and growing resistance to insecticide complicate pest control.

Geographic Range

H. assulta is distributed across three continents: Asia, Africa and Australia. Various countries in Asia include China, Korea, Thailand, and Japan. Environmental conditions, like temperature and geographic locality, can impact the fecundity of the moth.

Food Resources

H. assulta is a pest of red peppers (Capsicum frutescens ), tobacco (Nicotiatna tobacum), tomato, and onion. They showed feeding preference in eating red peppers over tobacco. Nutritional content, and the ability of the moth to digest and assimilate the nutrients all contribute to the development and survival of this moth.

Capsaicin

H. assulta is one of very few insects that can successfully feed on and damage plants containing capsaicin like hot peppers found in countries like Korea. They prefer to feed on plants of family Solanaceae, but also feed on Lycoperisicon, Nicotiana, Physalis, and Solanum. Studies show that long-term dietary exposure to capsaicin has a growth-stimulating effect on the larvae. Its unique tolerance to capsaicin may have allowed H. assulta to expand its host range.

Life Stages

Larvae

Hatching larvae feed on tender leaves near the site of oviposition. When it starts to undergo through instar, larvae feed on the fruit and flowers of the host, and can infest over the course of their development. Newly hatched larvae exhibit greatest preference for tender tobacco leaves, followed by pepper and tomato leaves. The success of H. assulta is significantly dependent on its larval diet. The sex ratio differs slightly depending on the type of food the larvae feed on. Larvae who feed on mainly tobacco have a higher female ratio compared to those which feed on mainly pepper. The H. assulta also preferred bush red pepper, tobacco, and tomato as a host plant. The preference for these crops may be correlated to the semiochemicals or allelochemicals of the host.

Adult

As an adult, the moth prefers to feed on tomato during oviposition, followed by tobacco and pepper. Larval diets also impact the number of eggs laid. Adults that fed as larvae on red peppers significantly laid more eggs compared to those that fed on tobacco. Feeding on red peppers also yielded shorter generation time, heavier pupae and lower larval and pupal mortality compared to feeding on tobacco.

Mating

Mating occurs during the early hours of the scotophase, however the mating peak in virgin females tended to advance with age. Unmated adults also were observed to live longer than mated adults.

Sex Pheromones

H. assulta is polymorphic, with at least two different populations responding to different ratios and blends of various sex pheromones. Gas chromatography and mass spectrometry identified nine compounds from female ovipositor washings including hexadecanal, (Z)-9-hexadecenal, (Z)-11-hexadecenal, hexadecyl acetate, (Z)-9-hexadecenyl acetate, (Z)-11-hexadecenyl acetate, hexadecan-l-ol, (Z)-9-hexadecen-l-ol, and (Z)-11-hexadecen-1-ol. The main pheromones that are used are two types of hexadecanals, (Z)-hexadecenal and (Z)-11 hexadecenal, and (Z)-9-hexadecenyl acetate. Field studies conducted in Korea, China, and Thailand also illustrated that a blend of (Z)-9-hexadecenal and (Z)-11-hexadecenal was sufficient for attraction, however the most attractive ratio of compounds varied by specific location. Moths found in Korea found that a 20:1 blend of (Z)-9-hexadecenal to (Z)-11-hexadecenal was most attractive. In Thailand, a 7.5:1 blend was most attractive for the moths. In China, the moths found the two different blends were equally attractive.

Females

Prior to mating, females protract and retract the terminal abdomen and vibrate the wings.

Release of Pheromones

The release of sex pheromones is mediated by the PBAN (pheromone biosynthesis activating neuropeptide), a phermonotropic factor that is synthesized in the suboesophageal ganglion and released into the hemolymph. Because PBAN can be produced independently of the moth's photoperiod, the circadian rhythm of pheromone production must be closely associated with PBAN release. Sex pheromones are only released during scotophase, and immediately releases the pheromones after synthesis. Maximum pheromone titer is from day one to day five, and then decreases.

The highest concentration of the major female sex pheromone, (Z)-9 hexadecenal (Z9-Z16:Al), is released in a distinct pattern over a 24-hour period or a circadian periodicity. This pattern is affected by age, photoperiod, and temperature. The daily rhythm of hormone production varies when the female moths are reared under 12 hours of light and 12 hours of darkness vs. continuous light conditions. The maximum pheromone titers in the gland corresponded to the peak calling activities, indicating that these events are synchronous and predictable. In constant darkness, these events were also synchronous. However, when placed in constant light, the H. assulta has two different patterns for pheromone release and calling behavior. Calling was suppressed, but the pheromone release was not. This suggests that these two behaviors are controlled by two different systems, and can somehow communicate to synchronize under normal conditions.

When moths are placed in constant light, there is a longer retention and slower decrease of Z9-16: AL in the pheromone gland. The moths may continue to produce the pheromone for a longer period of time, or that the degradation mechanism is inactive and the chemical may only decrease through release.

Calling Behavior

Calling behavior occurs when the ovipositor protrudes from the abdomen, and also demonstrates circadian periodicity. Calling is mostly performed after day three. There seems to be a time lag of a few days between the peaks of pheromone concentration release and calling activity, which also may suggest two different systems controlling these events.

Males

Mating behavior of adult males includes antennal movement, wing elevation and vibration, extension of hairs, and tapping of the female ovipositor, leading to copulation. Studies also indicate that sex pheromone release is also mediated by the PBAN.

Pest Control

H. assulta is considered to be one of the most destructive pests for various economic crops like tobacco and hot peppers. The moth has caused considerable economic damage to crops in Korea and China. The larvae usually feed on the fruit of host plants. Unlike other insects, it has a high tolerance for the toxicity of capsaicin. While conventional chemical insecticides are necessary for the control of these moths, the growing resistance of the moth and the detrimental effects of overusing these chemicals have drawn researchers to develop other methods of control. Furthermore, the larvae feed inside the fruit, and are therefore protected from the chemical sprays. Studies have illustrated that the type of plant is can impact and influence its response to insecticide. H. assulta that were fed on red peppers were more susceptible to certain insecticides such as fenvalerate, but became more resistant to chemicals like inoxacarb, phoxim and methomyl. Current research explores sex pheromone-mediated communication, a method of control aiming to interrupt moth communication. Also there is a need to understand the complex interactions among the insect, plant, and insecticide.