Adult
The adult is greyish brown with a 9-mm-long body and a wingspan of about 12-15 mm (Anonymous, 1983;Reid and Cuthbert, 1971). In males, upper (costal) two-thirds of forewings is light fuscous, sometimes partially ochre-tinged;sometimes mixed with whitish scales, and flecked with scanty small blackish dots. Lower one-third of the forewings is ochreous-white, the upper edge being nearly white, margined broadly with dark brown or black-brown. In females, the upper two-thirds of forewing is light ochreous or light grey-ochreous, the contrast not so pronounced between upper and lower portions in coloration, but the markings are like those of males. When wings are folded, three or four diamond-shaped areas formed by forewings are visible on the dorsal side when moth is at rest, hence the common name 'diamondback moth'. Moriuti (1986) gives details of wing venation and genitalia. The moths are weak fliers and can disperse, on average, only 13-35 m within a crop field (Mo JianHua et al., 2003). They are readily carried by the wind and can travel long distances, at 400-500 km per night (Chapman et al., 2002).
Egg
Colour: when disturbed, tiny adults fly from plant to plant. When at rest, three or four diamond-shaped areas formed by two forewings, are visible on the dorsal surface. Pale-green larvae with pale green to brown head capsules or brown pupae covered in white silken cocoons are present on plant parts damaged by P. xylostella.
Size: adult 10-12 mm long, fully-grown larva 10 mm long, pupa 5-6 mm long.
Behaviour: adults fly when disturbed. Larvae curl up when disturbed, or drop from the foliage to the ground.
Traps: adults are attracted to light traps. Adult males are attracted to sex pheromone which consists of three chemicals: (Z)-11-hexadecenal, (Z)-11-hexadecenyl acetate and (Z)-11-hexadecenyl alcohol (Chow et al., 1978). The yellow sticky traps can also be used to monitor populations in the field (Sivapragasam and Saito, 1986).
Food: Major host plants associated with the family Cruciferae with a few host plants in the family Capparidaceae (Idris, 1998;Tanaka et al., 1999).
Scouting Techniques in Oilseed rape
The count method, although often laborious, is currently the most accurate method of estimating P. xylostella population densities in oilseed rape. It involves performing counts of larvae in several locations throughout the field and determining the average population per unit area. Remove plants in an area of 0.1 m 2, beat them onto a clean surface, and count the number of larvae dislodged from the plants. Scout at least five locations per field and monitor crops at least twice weekly (Canola Council of Canada, 2014).
The action threshold in Canadian oilseed rape crops is 20-30 larvae/0.1 m 2 at the advanced pod stage. This works out to approximately two to three larvae/plant, given the plant population is about 100 plants/m 2 (Canola Council of Canada, 2014).
Sweep net sampling and trapping (e.g. sticky, pheromone and bowl traps) can be used to detect the presence and general abundance of P. xylostella in the field, but these tools alone may not provide a reliable estimate of larval density. Nevertheless, high counts in sweep sampling and trapping can prompt growers to use the more accurate 'count method' (Sarfraz et al., 2010;Canola Council of Canada, 2014).
In regions such as Canada where P. xylostella infestations are associated with annual migrations, pheromone traps coupled with wind trajectory models are useful tools to determine the size and timing of the moth flight.
Scouting Technique in Brassica Vegetables
In Brassica vegetable crops, the 'percent infested' threshold scouting technique is more efficient in detecting damaging pest populations as it avoids the need to remove plants and count pests and is relatively easy for growers to use (Berry, 2000). This technique is successfully used to scout several other insect and mite pests in commercial crops.
Various types of traps (e.g. sticky, pheromone, pitfall and bowl traps) can also be used to detect the presence and relative abundance of P. xylostella in the field.
Related invasive species
- Plutella xylostella
Related Farm Practice
- Travel
- Light
- Rest
- Damage
The diamondback moth (DBM) is one of the most studied insect pests in the world, yet it is among the 'leaders' of the most difficult pests to control. It was the first crop insect reported to develop resistance to microbial Bacillus thuringiensis insecticides, and has shown resistance to almost every insecticide, including the most recent groups such as diamide. DBM is a highly invasive species. It may have its origin in Europe, South Africa or East Asia, but is now present wherever its cruciferous hosts exist and is considered to be the most universally distributed Lepidoptera. It is highly migratory and wind-borne adults can travel long distances to invade crops in other regions, countries and continents. Immature stages also hitchhike on plant parts and can establish in new areas. DBM costs the global economy an estimated US$4 -5 billion annually, but its impacts on local biodiversity and habitats in exotic ranges are unknown.
The insect larva is a surface feeder and with its chewing mouthparts it feeds voraciously on the leaves leaving a papery epidermis intact. This type of damage gives the appearance of transluscent windows or 'shot holes' in the leaf blades. Insect larvae and, in many cases, pupae are found on the damaged leaves. In cases of severe infestation, entire leaves could be lost, leaving only the veins. The larvae nibble the chlorophyll-rich green areas of stems and pods and the damage shows from a distance as an unusual whitening of the crop. The damage is often first evident on plants growing on ridges and knolls in the field (Canola Council of Canada, 2014). Heavily damaged plants appear stunted and in most cases die.
In oilseed rape plants, larvae also feed on flower buds, flowers and young seed pods. The seeds within damaged pods do not fill completely and pods may shatter prematurely. Larvae also chew into pods and consume the developing seeds. Extensive feeding on the reproductive plant parts significantly reduces crop yields (Canola Council of Canada, 2014).
The natural host plant range of P. xylostella is limited to Brassicaceae which are characterized by having glucosinolates, sulfur-containing secondary plant compounds. Glucosinolates may be toxic to generalist insects, but DBM is known to rely on some of them for host location, oviposition and herbivory. Certain glucosinolates, cardenolides, plant volatiles, waxes, as well as host plant nutritional quality, leaf morphology and leaf colour, or a combination of these factors, may trigger reproductive and feeding activities of DBM (Sarfraz et al., 2006 and references therein).
Cruciferous weeds serve as alternate hosts (Sarfraz et al., 2011). For instance, the wind-borne moths can arrive in parts of the oilseed rape growing areas in Canada from the southern USA early enough that many of the rape crops will not have emerged yet (Canola Ccouncil of Canada, 2014). In these situations cruciferous weeds become important alternate 'bridge' hosts.
Some populations have also been found to infest non-cruciferous plants (see List of Hosts). However, host plant shift from feeding on crucifers to feeding on non-crucifers may depend on geographical populations. For example, a Kenyan population of P. xylostella adapted to sugar snap peas (Löhr and Gathu, 2002) whereas a Canadian population, despite of multiple attempts, could not survive on peas in the laboratory (Sarfraz, unpublished data).
For further information on hosts, see Sarfraz et al. (2006, 2010, 2011) and references therein, and Sakakibara and Takashino (2004).