Plutella xylostella Linnaeus Lepidoptera Plutellidae

Natural History

Distribution. Diamondback moth is probably of European origin but has become rather cosmopolitan, and is now found throughout the Americas and in Europe, Southeast Asia, Australia, and New Zealand. It was first observed in North America in 1854, in Illinois, but spread quickly. It was found in Massachusetts and Maryland by 1870, had spread to Florida and the Rocky Mountains by 1883, and was reported from British Columbia by 1905. In North America, dia-

mondback moth is now recorded everywhere that cabbage is grown, even as far north as Canada's Northwest Territories. By virtue of its ability to feed on cruciferous weeds, diamondback moth is sometimes abundant even in some areas where cruciferous crops do not occur. However, it is highly dispersive, and is often found in areas where it cannot successfully overwinter, including most of Canada (Beirne, 1971).

Host Plants. Diamondback moth attacks only plants in the family Cruciferae. Virtually all cruciferous vegetable crops are eaten, including broccoli, Brussels sprouts, cabbage, Chinese cabbage, cauliflower, col-lards, kale, kohlrabi, mustard, radish, turnip, and watercress. Not all are equally preferred, however, and collards usually chosen by ovipositing moths relative to cabbage. Several weeds are important hosts, especially early in the season before cultivated crops are available. Yellow rocket, Barbarea vulgaris; shep-herdspurse, Capsella bursa-pastoris; pepperweed, Lepi-dium spp.; and wild mustards, Brassica spp. are commonly cited as important weed hosts.

Natural Enemies. A comprehensive analysis of diamondback moth mortality factors was conducted in Ontario by Harcourt (1960, 1963b). Large larvae, pre-pupae, and pupae often were killed by the parasi-toids Microplitis plutellae (Muesbeck) (Hymenoptera: Braconidae), Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae), and Diadromus subtilicornis (Gravenhorst) (Hymenoptera: Ichneumonidae). All are specific on P. xylostella. In Ontario, D. insulare was considered most important except during diamondback moth population outbreaks when the other species assumed greater importance. More recent studies conducted in Quebec are consistent with the Ontario studies (Godin and Boivin, 1998b). Diadegma insulare was also important in California (Oatman and Platner, 1969; Kennedy and Oatman, 1976). Nectar produced by wildflowers is important in determining parasitism rates by D. insulare (Idris and Grafius, 1995). Egg parasites are poorly known. However, a Trichogramma egg parasitoid is reported in Japan (Wakisaka et al., 1992) and also from Florida (Leibee, pers. comm.). Fungi, granulosis virus, and nuclear polyhedrosis virus sometimes occur in high density diamondback moth larval populations.

Weather. Harcourt (1960, 1963b) and Wakisaka et al. (1992) found that a large proportion of young larvae were often killed by rainfall. However, the most important factor determining population trends was reported to be adult mortality. Adult survival was thought to be principally a function of weather, though this hypothesis has not been examined rigorously.

Life Cycle and Description. Total development time from the egg to pupal stage averages 25-30 days, depending on weather (range about 17-51 days). In Ontario, diamondback moth is present from May to October, but is most abundant in July and September. There are 4-6 generations annually, but discrete broods are not apparent (Harcourt, 1957). In Quebec, Godin and Boivin (1998a) reported adults and eggs beginning in early June, and estimated 3-4 generations annually. In Colorado, the number of annual generations is estimated to be seven, and overwintering survival is positively correlated with the abundance of snowfall (Marsh, 1917). There is continuous breeding in the southern states, so the number of generations is likely 12-15 per year.

  1. Diamondback moth eggs are oval and flattened, and measure 0.44 mm long and 0.26 mm wide. They are yellow or pale-green, and are deposited singly or in small groups of 2-8 eggs in depressions on the surface of foliage, or sometime on other plant parts. Females may deposit 250-300 eggs early in the year, but the number decreases in later generations by 90%; average total production is probably 150 eggs. Development time averages 5.6 days (range 4-8 days).
  2. Diamondback moth has four instars. Average and range of development time is about 4.5 (3-7), 4 (2-7), 4 (2-8), and 5 (2-10) days, respectively. Throughout their development, larvae remain quite small and active. If disturbed, they often wriggle violently, move backward, and spin down from the plant on a strand of silk. Overall length of each instar rarely exceeds 1.7, 3.5, 7.0, and 11.2 mm, respectively, for instars 1-4. Mean head capsule widths for these instars are about 0.16, 0.25, 0.37, and 0.61 mm. Larval body form tapers at both ends, and a pair of prolegs protrudes from the posterior end, forming a distinctive "V". The larvae are colorless in the first instar, but thereafter they become green. The body bears relatively few hairs, which are short in length, and most are marked by the presence of small white patches. There are five pairs of prolegs. Initially, the feeding habit of first instar larvae is leaf mining, though they are so small that the mines are difficult to notice. The larvae emerge from their mines at the conclusion of the first instar, molt beneath the leaf, and thereafter feed on the lower surface of the leaf. Their chewing results in irregular

Diamondback moth larva.

Diamondback moth larva.

patches of damage, and the upper leaf epidermis is often left intact. (See color figure 71.)

  1. Pupation occurs in a loose silk cocoon, usually formed on the lower or outer leaves. In cauliflower and broccoli, pupation may occur in the florets. The yellowish pupa is 7-9 mm long. The duration of the cocoon averages about 8.5 days (range 5-15 days), but during this time the insect is in the pre-pupal rather than the pupal stage.
  2. The adult is a small, slender, grayish-brown moth with pronounced antennae. It is about 6 mm long, and marked with a broad cream or light-brown band along the back. The band is sometimes constricted to form one or more light-colored diamonds on the back, which is the basis for the common name of this insect. When viewed from the side, the tips of the wings can be seen to turn upward slightly. Moths usually mate at dusk, immediately after emergence from the cocoon. Flight and oviposi-tion take place from dusk to midnight, and moths can be found feeding at blossoms on nectar. Adult males and females live about 12 and 16 days, respectively, and females deposit eggs for about 10 days. The moths are weak fliers, usually flying within 2 m of the ground, and not flying long distances. However, they are readily carried by the wind. The adult is the overwintering stage in temperate areas, but moths do not survive cold winters, as is found in most of Canada. They routinely re-invade these areas each spring, evidently aided by southerly winds (Smith and Sears, 1982).

Detailed biology of diamondback moth can be found in Marsh (1917) and Harcourt (1955, 1957, 1963b). A survey of the world literature was published by Talekar et al. (1985). Rearing techniques were provided by Biever and Boldt (1971) and Liu and Sun (1984). Chow et al. (1974) identified two major components of the sex pheromone, and Chisholm et al. (1983) evaluated a four-component mixture, but the exact blend remains undetermined.

Diamondback Moth Adult
Adult diamondback moth.
Frugiperda
Adults of diamondback moth.

Damage

Damage is caused by larval feeding. Although the larvae are very small, they can be quite numerous, resulting in complete removal of foliar tissue except for the leaf veins. They are particularly damaging to seedlings, and may disrupt head formation in cabbage, broccoli, and cauliflower. The presence of larvae in florets can result in complete rejection of produce, even if the level of plant tissue removal is insignificant.

Diamondback moth was long considered a relatively insignificant pest. Its impact was overshadowed by such serious defoliators as imported cabbageworm, Pieris rapae (Linnaeus), and cabbage looper, Tricho-plusia ni (Hübner). However, in the 1950s the general level of abundance began to increase, and by the 1970s it became troublesome to crucifers in some areas. Although this shift in abundance has been attributed to increased availability of alternate weed hosts or destruction of parasitoids, insecticide resistance was long suspected to be a component of the problem. This was confirmed in the 1980s as pyrethroid insecticides began to fail, and soon thereafter virtually all insecticides were ineffective (Leibee and Capinera, 1995). Relaxation of insecticide use, which can be implemented by use of thresholds to trigger applications, rotation with Bacillus thuringiensis and particularly elimination of pyrethroid use, can return diamondback moth to minor pest status by favoring survival of parasitoids.

Management

Sampling. Populations are usually monitored by making counts of larvae, or by the level of damage. In Texas, average population densities of up to 0.3 larvae per plant are considered to be below the treatment threshold (Kirby and Slosser, 1984; Cartwright et al., 1987). In Florida and Georgia, treatment is recommended only when damage equals or exceeds one hole per plant (Workman et al., 1980). When growers monitor fields and subscribe to these treatment thresholds rather than trying to prevent any insects or damage from occurring in their fields, considerably fewer insecticide applications are needed to produce a satisfactory crop. Harcourt (1961) studied the distribution of various life stages on cabbage, and recommended a minimum plant sample size of 40-50 except for the egg stage, where 150 plants should be examined for accurate population estimates. Shelton et al. (1994) compared the benefits of sequential and variable-intensity sampling for diamondback moth management, and recommended the latter as being more reliable and requiring fewer samples.

Pheromone traps can be used to monitor adult populations, and may predict larval populations 1121 days later. However, because of variation among locations, each crop field requires independent evaluation (P. B. Baker et al., 1982).

Insecticides. Protection of crucifer crops from damage often requires application of insecticide to plant foliage, sometimes as frequently as twice per week. However, resistance to insecticides is widespread, and includes most classes of insecticides including some Bacillus thuringiensis products. Rotation of insecticide classes is recommended, and the use of B. thuringiensis is considered especially important because it favors survival of parasitoids. Even B. thuringiensis products should be rotated, and current recommendations generally suggest alternating the kurstaki and aizawa strains because resistance to these microbial insecticides occurs in some locations (Tabashnik et al., 1990). Mixtures of chemical insecticides, or chemicals and microbials, are often recommended for diamondback moth control (Leibee and Savage, 1992). This is due partly to the widespread occurrence of resistance, but also because pest complexes often plague crucifer crops, and the insects vary in susceptibility to individual insecticides.

Sex pheromone, though not usually considered to be an insecticide, can also be used as a chemical crop protectant. Continuous release of pheromone has been investigated as a technique to suppress diamondback moth mating activity. McLaughlin et al. (1994) reported that the number of insecticide treatments could be reduced from 13-15 to only three when crops were grown in the continuous presence of diamondback moth pheromone.

Cultural Practices. Rainfall has been identified as a major mortality factor for young larvae, so it is not surprising that crucifer crops with overhead sprinkle irrigation tend to have fewer diamondback moth larvae than drip or furrow-irrigated crops. Irrigation also tends to disrupt oviposition. Best results were obtained with daily evening applications (McHugh and Foster, 1995).

Crop diversity can influence abundance of diamondback moth. Larvae generally are fewer in number, and more heavily parasitized, when crucifer crops are interplanted with another crop or when weeds are present. This does not necessarily lead to reduction in damage, however (Bach and Tabashnik,

1990). Surrounding cabbage crops with two or more rows of preferred hosts such as collards and mustard can delay or prevent the dispersal of diamondback moth into cabbage crops (Srinavasan and Moorthy, 1992).

Crucifer transplants are often shipped long distances before planting, and diamondback moth may be included with the transplants. In the United States, many transplants are produced in the southern states, and then moved north as weather allows. Cryptic insects such as young diamondback moth larvae are sometimes transported, and inoculated in this manner. The transport of insecticide-resistant populations also may occur (Wyman, 1992). Every effort should be made to assure that transplants are free of insects before to planting.

Host-Plant Resistance. Crucifer crops differ somewhat in their susceptibility to attack by diamondback moth. Mustard, turnip, and kohlrabi are among the most resistant crucifers, but resistance is not as pronounced as it is for imported cabbageworm and cabbage looper (Radcliffe and Chapman, 1966). Varieties also differ in susceptibility to damage by diamondback moth, and a major component of this resistance is the presence of leaf wax. Glossy varieties, lacking the normal waxy bloom and therefore green rather than grayish green, are somewhat resistant to larval feeding (Dickson et al., 1990; Stoner, 1990; Eigenbrode et al.,

1991). Larvae apparently spend more time searching, and less time feeding, on glossy varieties. Glossy varieties also tend to have fewer imported cabbageworm larvae and cabbage aphids, but more cabbage flea beetles.

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