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Lepidopterous Insect
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Due to their often bright colours and association with warm sunny days, butterflies have tended to catch the popular imagination over the centuries, more than any other insects. They can even be found adorning some ancient Egyptian tombs.
Many museums, societies and some individual entomologists maintain a reference collection of preserved specimens of butterflies and moths (and other insect orders) for reference or study. These specimens are kept in either glass topped drawers in a cabinet, or in purpose made storage boxes.
However, it should be remembered that many insect species are declining, and most people now prefer to study the living insects than to collect dead specimens. Digital photography has lessened the need to maintain reference collections, thereby releasing more time to watch and study these fascinating creatures.
Crop protection companies have developed a broad range of very effective insecticides for the control of lepidopteran pests. Unfortunately, as a consequence of the misuse or overuse of these insecticides, many species have developed resistance. Populations of Plutella xylostella, for example, have developed resistance to virtually every insecticide used against them. Additionally, there are numerous other lepidopteran species prone to resistance development.
Lepidoptera (moths and butterflies) are thesecond most diverse pest insect order outnumbered only by thebeetles. There is hardly any cultivated plant that is notattacked by at least one lepidopteran pest. As pollinators ofmany plants, adult moths and butterflies are usually beneficialinsects that feed on nectar using their siphoning proboscis. Thecaterpillars however almost always have chewing mouthparts thatare suitable for feeding on various parts of a plant. Mostcaterpillars are defoliators or miners of succulent planttissues.
Severe pests of seedlings in nurseries are the larvae of Noctuidae,called cutworms or army worms, for instance Agrotisipsilon. The immature stages live hidden in soil. Thecaterpillars emerge from the soil and feed on their host duringthe night. Typically, a young plant is neatly cut off right abovethe soil. Partial but complete defoliation or bending of theseedling is also common. If a cutworm problem is severe orpersists in a nursery, the seedlings have to be protected by theapplication of a suitable insecticide.
As with many polyphagous insects, although the host range is potentially very broad, the insect has definite food preferences, and the suitabilities of the food plants vary as well. Greenberg et al. 2001 compared growth of beet armyworm larvae on three crops (cabbage, cotton, pepper) and two weeds (redroot pigweed, sunflower). They reported that the level of relative consumption was cabbage, followed by pepper, sunflower, cotton, and pigweed. Relative growth, judged by pupal weights, was greatest on pigweed, equal for cotton, cabbage, and pepper, and least on sunflower. The larvae also matured fastest on pigweed. The authors interpreted these results to indicate that pigweed was nutritionally superior, and that cabbage was the poorest host
Larvae feed on both foliage and fruit. In Florida it is regarded as a serious defoliator of flower crops and cotton, though much of the injury is induced by insecticide use that interferes with natural enemy activity. Young larvae feed gregariously and skeletonize foliage. As they mature, larvae become solitary and eat large irregular holes in foliage. They also burrow into the crown or center of the head on lettuce, or on the buds of cole crops. As a leaf feeder, beet armyworm consumes much more cabbage tissue than the diamondback moth, Plutella xylostella (Linnaeus), but is less damaging than the cabbage looper, Trichoplusia ni (Hübner) (East et al. 1989), as the latter grows to be a much larger caterpillar.
Pheromone traps can be used to detect the presence of adult beet armyworm. Visual sampling for damage and larvae, combined with an action threshold of 0.3 larvae per plant, was used successfully on cabbage in south Texas to determine the need for crop treatment with insecticides (Cartwright et al. 1987). Regular monitoring of crops, probably about twice per week, is recommended because adults frequently invade from surrounding crops or weeds.
Several insect pathogens may prove to be useful for suppression of beet armyworm. A nuclear polyhedrosis virus isolated from beet armyworm is fairly effective as a bioinsecticide under greenhouse conditions, where inactivation by ultraviolet light in sunlight is not a severe problem. The fungus Beauveria bassiana has the same attributes and limitations. Entomopathogenic nematodes (Rhabditida: Steinernematidae and Heterorhabditidae) successfully infect both larvae and adults of beet armyworm, though it is difficult to attain suppression of foliage-feeding insects under field conditions because the nematodes are favored by moisture. Thus, the nematodes are more effective at killing the larvae when they drop to the soil to pupate; the prepual stage is more susceptible than the pupal stage.
Ostrinia nubilalis (European Corn Borer or ECB) is maybe the most famous lepidopterous corn pest. It originates from Europe but has also invaded North America and is known to infest other crops as well such as sorghum, peppers, beans, tomatoes, cannabis and sometimes potatoes. It thrives in countries of the Mediterranean basin and it can withstand the low temperatures of Northern European countries such as Germany, Poland, Hungary, Czech Republic etc. It completes 1-2 generations in the North and 2-3 in the South of Europe.
Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) is another lepidopterous pest of corn and is rather opportunistic as it prefers to infest other crops, at least in Greece, such as cotton, okra, tomato and pepper. However, whenever its population on these crops is very dense it seems that it \"switches\" to other unexploited hosts which are abundant in its habitat. And corn is one of these crops (photograph below).
The fact that Ostrinia, Sesamia and Helicoverpa appear late in the growing season (usually from July or even from August onward) is of cornerstone importance because with an early sowing and a short/medium biological cycle of maize hybrid the crop can escape from devastating insect attacks. However, controlling these pests in corn is a rather difficult task when infestation occurs on late sown corn or at a stage when corn is too tall. In this case, insecticide application requires special equipment which is not available in Greece due to the small size of farms.
There are quite a few effective insecticides in the market today which have specific technical profiles and therefore certain advantages and disadvantages. Most of them act through ingestion and/or direct contact. However, apart from its intrinsic toxicity to the pest, the most important property of an insecticide for the control of corn lepidoptera seems to be its residual effect because it will have to deal with multiple pest stages of varying sensitivity and overlapping generations.
What is more, a good knowledge of the biological cycle and behavior of these pests helps a lot in a control strategy planning. A case of a corn field in Izmir, Turkey, infested by both Ostrinia and Sesamia back in Aug 2018 is a good example: The farmer sprayed an insecticide and the timing was perfect for Ostrinia which was at the egg hatching stage. However Sesamia caterpillars were quite advanced and mostly \"hidden\" inside corn ears, so they continued their destructive activity practically unaffected.
The cabbage looper is found throughout much of the world where crucifers are cultivated, and during the summer months can be found throughout most of the USA; however, overwintering in the USA apparently occurs only in the southernmost states. It is somewhat erratic in occurrence, typically very abundant one year, and then scarce for two to three years. This is likely due to the residual effects of a nuclear polyhedrosis virus, which is quite lethal to this insect. The cabbage looper is highly dispersive, and adults have sometimes been found at high altitudes and far from shore. Flight ranges of approximately 200 km have been estimated.
The number of generations completed per year varies from two to three in Canada, five in North Carolina, to five to seven in California. The generations overlap considerably, and therefore are indistinct. Development time (egg to adult) requires 18 to 25 days when insects are held at 32 to 21C, respectively (Toba et al. 1973), so at least one generation per month could be completed successfully under favorable weather conditions. There is no diapause present in this insect, and although it is capable of spending considerable time as a pupa, it does not tolerate prolonged cold weather. It reinvades most of the United States and all of Canada annually after overwintering in southern latitudes. The lower limit for development is about 10 to 12C, and temperatures of 40C of higher can also be lethal to some stages. In Florida, continuous activity and reproduction occur only south of Orlando. The remainder of Florida and the portion of Georgia south of Byron, as well as southeast South Carolina, have intermittent adult activity during the winter months, depending on weather. All points north of this have no winter activity.
Sampling: Various sampling strategies have been developed for cabbage looper, and many approaches include consideration of the other crucifer-feeding caterpillars. Fixed sample units of at least 40 plants are sometimes recommended. However, sequential sampling (Shepard 1973) and variable intensity sampling (Hoy et al. 1983) protocols have been developed to minimize the amount of sampling required to make appropriate management decisions. Dornan et al. (1995) recommended a binomial (presence-absence) approach because it eliminated counting and insect identification. 59ce067264
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