Diamondback moth
The diamondback moth, sometimes called the cabbage moth, is a moth species of the family Plutellidae and genus Plutella. The small, grayish-brown moth sometimes has a cream-colored band that forms a diamond along its back. The species may have originated in Europe, South Africa, or the Mediterranean region, but it has now spread worldwide.
The moth has a short life cycle, is highly fecund, and is capable of migrating long distances. Diamondback moths are considered pests as they feed on the leaves of cruciferous crops and plants that produce glucosinolates. However, not all of these plants are equally useful as hosts to the moth. Because of this, studies have suggested using wintercress as a trap crop around agricultural fields because diamondback moths are highly attracted to that plant but their larvae fail to survive when eggs are laid on it.
Originally, pesticides were used to kill the moths but diamondbacks have developed resistance to many of the common chemicals. For this reason, new biological and chemical controls, as well as different planting methods, are being pursued to reduce the destruction caused by the moths.
Description
This small moth is colored gray and brown. It can potentially identified by a cream-colored band that may be present in the shape of a diamond on its back. The diamondback moth has a wingspan of about 15 mm and a body length of 6 mm. The forewings are narrow, brownish gray and lighter along the anterior margin, with fine, dark speckles. A creamy-colored stripe with a wavy edge on the posterior margin is sometimes constricted to form one or more light-colored diamond shapes, which is the basis for the common name of this moth. The hindwings are narrow, pointed toward the apex, and light gray, with a wide fringe. The tips of the wings can be seen to turn upward slightly when viewed from the side. The antennae are pronounced.The adults of this species are visually identical to the adults of the New Zealand endemic moth ''Plutella antiphona.''
Geographic range
The diamondback moth has a global distribution and is found in Europe, Asia, Africa, the Americas, Australia, New Zealand, and the Hawaiian Islands. It is said by some experts to be the most widely distributed of all Lepidoptera, but despite tremendous interest in limiting the damage it causes, the actual available data is inadequate. It probably originated in Europe, South Africa, or the Mediterranean region, but the exact migration path is not known. However, in North America it was observed in Illinois in 1854, and then found in Florida and the Rocky Mountains by 1883. Although diamondback moths cannot overwinter effectively in cold climates, it was found in British Columbia by 1905 and is now present in several Canadian regions.Parental care
Oviposition
Diamondback moths prefer the cabbage plant, from the plant species Brassica oleracea, as their host plant. The females lay eggs only on the leaves of the cabbage and do not discriminate between young and more developed leaves. However, females are more likely to deposit their eggs on a host with larval infestation. It is not fully known why females do not choose the uninfested host, but it is thought that a specific, attractive odor is emitted by the infested host.Female diamondback moths use both gustatory and olfactory stimuli to determine where to lay their eggs. When both stimuli are available, more eggs are deposited. If gustatory stimuli or both gustatory and olfactory signals are absent, female moths will not lay their eggs. However, if only olfactory signals are absent, oviposition will continue.
Host plant learning and selection for egg laying
Host plants
Host plant selection is crucial because diamondbacks spend the majority of their life near their host plant. The diamondback moth lays its eggs only on plants in the family Brassicaceae. Nearly all cruciferous vegetable crops are attacked, but some are favored over others.These include
- Broccoli
- Brussels sprouts
- Cabbage
- Chinese cabbage
- Cauliflower
- Collard greens
- Kale
- Kohlrabi
- Mustard
- Radish
- Watercress
Odor
Different behaviors occur before a female diamondback moth deposits her eggs. While virgin and mated females both have the same sensitivity to a host plant's odor, pregnant diamondback females are more strongly drawn and sensitive to it because they are in search of a place to lay their eggs.Diamondbacks are nocturnal and use their olfactory system to discover the host plant odor. Additionally, in order to search for the host odor, they rotate their antennas. When the host odor is not present or in low concentrations the moth spends more time rotating its antennas. A moth has increased antennal rotation activity when it is near an uninfested host when compared to an infested host which indicates that the damaged host leaves emit a stronger odor.
Taste and touch
occurs when the moth hits its antennae on the leaf. This behavior is likely used to taste the host site. Only after antennation will the moth sweep its ovipositor across the site of deposition in order to gather more information about the host. Because the female moths lay their eggs one at a time and prefer crevices, they search for grooves on the leaves. The crevices may offer protection and easy access to food sources. However, grooves on leaves do not determine when oviposition occurs, but they may play a higher role in egg placement.Life cycle
Eggs
The eggs are oval and flattened, measuring 0.44 mm long and 0.26 mm wide. They are yellow or pale green at first, but darken later. They are laid singly or in groups of two to eight eggs in depressions on the surface of leaves. Females may deposit up to 300 eggs in total, but average production is probably half that amount. The larvae emerge from the eggs in about six to seven days.Larvae
The larvae have four instars, each with an average development time of about four days. The larval body form tapers at both ends. The larvae have a few short black hairs and are colorless in the first instar, but pale or emerald green with black heads in later instars. Of the five pairs of prolegs, one protrudes from the posterior end, forming a distinctive "V". The larvae are quite active, and when disturbed, may wriggle violently, move backward, and spin a strand of silk from which to dangle.The feeding habit of the first instar is leaf mining, although they are so small, the mines are difficult to detect. The larvae emerge from these mines to moult and subsequently feed on the lower surface of the leaf. Their chewing results in irregular patches of damage, though the upper leaf epidermis is often left intact. These irregular patches are called window panes.