|Crop Knowledge Master|
Plutella xylostella (Linnaeus)
Ronald F.L. Mau, Extension Entomologist
Jayma L. Martin Kessing, Educational Specialist
Department of Entomology
Updated by: J.M. Diez April 2007
Host plants include both cultivated and wild plants of the family Cruciferae, as well as several ornamentals, such as wallflower, candytuft, stocks, and alyssum. Cultivated crops that are attacked include broccoli, Brussels sprouts, cabbage, cauliflower, Chinese broccoli, Chinese cabbage, flowering white cabbage, head cabbage, mustard cabbage and watercress. Weed hosts, such as mustard and radish, are important reservoir hosts for the species.
The diamondback moth is a cosmopolitan species that probably originated in the Mediterranean region. It is found over much of North America, the southern portion of South America, southern Africa, Europe, India, Southeast Asia, New Zealand, and parts of Australia (Hardy, 1938). Accidentally introduced from Europe, it was first reported in North America in Illinois in 1854 and from western Canada in 1885 (Harcourt, 1962). It is now present throughout the U. S. and in every province of Canada. In Hawaii, it was first reported on Hawaii in 1892, and is now present on all islands.
The first instar sometimes feeds in the spongy plant tissue beneath the leaf surface forming shallow mines that appear as numerous white marks. These mines are usually not longer than the length of the body. The larvae are surface feeders in all subsequent stages. These larvae feed on the lower leaf surface 62-78% of the time, chewing irregular patches in the leaves (Harcourt, 1957). All the leaf tissues are consumed except the veins. On some leaves, the larvae feed on all but the upper epidermis creating a "windowing" effect. The last stage larva is a voracious feeder; it causes more injury than the first three larval instars.
Life stages of the diamondback moth are the egg, four larval instars, the pupa and adult. Eggs. larvae and pupae occur on the host. Adults occur on the host or on other plants adjacent to the crop. Adults are nocturnal in behavior. The ratio of the number of females to number of males is 1 to 1
At 60ûF the life cycle from egg to adult averages 27 days. Egg incubation was completed in 6 days. The larval period was completed in 14 days, and the pupal period reequired 7 days. At this temperature there could be as many as 14 generations per year (Ho, 1965).
Development is considerably more rapid at higher temperatures. At 80ûF the life cycle averaged about 11 days. Average egg incubation was 2 days. Larval development was completed in 6 days, and pupal development required 3 days. At this temperature there could be as many as 30 generations per year (Ho, 1965).
Continuous reproduction occurs in Hawaii and the southernmost parts of the United States where there are many overlapping generations throughout the year. There are two to six generations each year in temperate regions. Moths overwinter under old plant remnants. The time required to complete a generation varies from 18 to 51 days, and averages about 25 days during the summer in the U.S. mainland.
Eggs are laid singly or in groups of 2 to 8 on the upper or undersides of the leaves. They are frequently deposited in the hollows along the vein, on the young stems or on petioles. The small (1/40 x 1/80 inch), flat, oval-shaped eggs are shiny yellow when first laid. They have a finely reticulated surface (Ho, 1965). Just before hatching, the egg darkens and the young larva can be seen coiled beneath the chorion or egg shell (Harcourt, 1957). Less than 2% of eggs are infertile (Harcourt, 1957). Eggs hatch in 2 to 8 days.
On watercress, eggs are deposited on upper and lower leaf surfaces and on petioles. They are most often deposited on newly expanded leaves.
There are 4 larval stages (instars). Caterpillars are active, slender, green worms with microscopic hairs. They are approximately 1/3 inch in length when full grown. The heads of the first and second larval stages are black in color and distinct from the green to brown colored heads of the third and fourth stage larvae. The larval period varies from 6 to 30 days.
On young cabbage and on watercress, early larval instars migrate to the growing point and cause extensive feeding injury. The last larval instar is usually found close to the growing point but can be found on older leaves. The damage caused by the larvae feeding is often very severe and in most areas good quality cabbages cannot be grown unless the insect is controlled.
Larvae may also feed on other portions of the plant. Mature larvae feed on the florets of broccoli and cauliflower and bore into cabbage heads and Brussels sprouts. If larvae feed on the inner leaves of young cabbage plants before the heading stage (precupping) the cabbage plant develops several deformed and unmarketable heads.
When mature, larvae spin an open network, silken cocoon, 1/4 inch in length by 3/50 inch at its broadest width (Ho, 1965). Pupae usually occur on the undersurface of leaves and in other protected areas on the plant. The pupal period averages 8 days.
Adults are small, grayish moths, approximately 1/3 inch in length. When the wings are folded, the moths have a line of three diamond-shaped markings that occur along the middle of the back. Males and females are the same size.
Females live from 7 to 47 days, averaging 16.2; and males live from 3 to 58 days, averaging 12.1. The number of eggs laid per female may range from 18 to 356, the mean being 159. Oviposition normally begins on the day of emergence lasting about 10 days, the peak occurring on the first night of oviposition except when temperature at sunset is below 66ûF (Harcourt, 1957). The number of eggs produced by a single female is influenced by photoperiod, temperature, and age or condition of larval food (Harcourt, 1957).
The newly hatched larvae crawl to the under surface of the leaf and may shallowly bore into the leaf and feed on tissue beneath the leaf surface. The remaining three larval stages feed on the surface, consuming all the leaf tissue except the veins and sometimes the upper epidermis.
Larvae wriggle backwards rapidly when disturbed and may drop from the leaf suspended by a silken thread to remain suspended temporarily or drop to a lower leaf. When the disturbance has passed, the larvae "climb" the silken thread back to the leaf and crawl away, leaving a coil of silk on the leaf.
Larvae are susceptible to drowning; the first instars are the most vulnerable. An average mortality of 56% caused by rainfall was reported by Harcourt (1957). Mortality due to rainfall is affected by temperature (colder temperatures cause higher mortality) and the intensity of rainfall (the harder the rain the higher the mortality).
Mating begins on the day of emergence at dusk while moths are resting on the plants. Mating will last about 1 hour, if the couple is disturbed during this period the female will drag the male to a more sheltered location on the plant. Egg laying begins shortly after dusk and peaks about two hours later, few eggs are laid after midnight.
Diamondback moths are nocturnal fliers, flight activity peaks at 7 PM and 7 AM (1 hr after sunset and 1 hr before sunrise) (Danthanarayana, 1984). Goodwin and Danthanarayana (1984) reported that female and male flight activity peaked about 2 and 4 hours after sunset, respectively, but flight activity continued through the night. There was a pronounced increase in adult flight activity that was initiated at sunset by a combination of factors. The predisposing cue was failing light prior to sunset. Other factors that influenced flight was temperature and wind speed. Temperatures below 44.6ûF limit flight activity. Flight activity was greatest at wind speeds below 2.25 MPH (Goodwin and Danthanarayana, 1984)
Adults are inactive during the day. If disturbed during the day, they fly erratically. Adults feed on flowers of the Brassicae family just before dusk. They crawl and fly short distances in a jump-like fashion, however they are easily carried by the wind for great distances. On windless days they travel no more than 10 to 12 feet horizontally and seldom higher than 5 feet above ground. Studies in Canada of flight behavior found more females than males flew during the first two hours of the night. This ratio reversed as the night progressed (Harcourt, 1957). Females are less active and rarely fly at temperatures below 55ûF and when wind velocities exceed 4 MPH (Harcourt, 1957).
Migrations and activity of diamondback moth may be associated with lunar cycles. Flight activity peaks shortly after new moon, around full moon and shortly before new moon (first and last quarters of moon and full moon). This relationship to the lunar cycle is independent of environmental conditions such as temperature, percent relative humidity, time of sunset and sunrise, rainfall and wind speed (Danthanarayana, 1986).
The diamondback moth is a key pest of cabbage and other crucifers. It is difficult to control because of genetic resistance to insecticides. Common pests associated with the diamondback moth are the cabbage aphid (Brevicoryne brassicae), the green peach aphid (Myzus persicae), the imported cabbageworm (Artogeia rapae), the cabbage looper (Trichoplusia ni), cutworms (Agrotis spp. and Peridroma saucia), the beet armyworm (Spodoptera exigua), and the imported cabbage webworm (Hellula undalis). Control by only biological or chemical means is difficult to achieve and a combination of these and other tactics are needed.
In 1992 the diamondback moth on Hawaiian vegetable farms became resistant to pyrethroid insecticides. In order to achieve adequate levels of control, growers must now rely on bacterial insecticides, variety selection, parasitoids, and planting schedules. We estimate that field life of bacterial insecticides is about 2-3 days. In order to effectively utilize bacterial insecticides against diamondback moth, growers must time applications so that treatment is made soon after eggs hatch. This will asssure that the insecticide is active when larvae are feeding. Another important consideration is coverage. Bacterial insecticides must be ingested to kill the larvae. Therefore, the insecticidal sprays must be applied to achieve full coverage of upper and lower leaf surfaces.
From the economic point of view, the larval stage is the one responsible for the damage, and a mortality factor operating before this stage is more important than one that operates after it (Harcourt, 1957). Larvae, especially the younger stages, are very susceptible to drowning. During periods of rainy weather and high humidity when there are droplets of water in the area more than half of the first three larval stages may perish by drowning (Waterhouse, 1987). Intermittant overhead irrigation of watercress provides effective economic control of diamondback moth on Oahu during periods of normal tradewind weather. The sprinklers are operated from early morning to approximately 9 pm. Control is achieved in 4-6 weeks after begining the program. Unfortunately, diseases become a problem when air circulation is poor. Growers typically turn off their systems during periods of southerly winds to reduce the impact of diseases.
Discing under crop debris immediately after harvest help to prevent the buildup of diamondback moth and subsequent migrations to younger plants in adjacent fields (Flint, 1985).
Parasitoids of the diamondback moth occur naturally and can reduce the next egglaying generation. Parasites and predators play a dominant role in the biological control of diamondback moth. Of the over 90 parasites of the diamondback moth recorded from various parts of the world, several of these are known to almost always occur with the pest and be the dominant natural enemies. High levels of parasitism have been reported to maintain low levels of diamondback moth in many parts of the world (Waterhouse, 1987). In food crops where damage to vegetables must be minimal, chemical control has not been eliminated by biological control (Harcourt, 1963).
In Hawaii several parasites have been liberated to contribute to the control of the diamondback moth. Cotesia (Apanteles) plutellae, a native of Europe that attacks the first three larval stages of the diamondback moth, is widespread after a reintroduction in 1980. In cooler weather, Diadegma insulare is widespread and an important controlling factor. Other liberated parasites include Brachymeria boranensis (1953), Diadromus collaris (1983), Tetrastichus nr. sokolowskii (1953), and Trichogramma chilonis (1984) (Waterhouse, 1987). Other parasitoids that have been reared from the diamondback moth in Hawaii include Pristomerus hawaiiensis Perkins, and Chelonus blackburni Cameron (Johnson, et al, 1988).
High levels of resistance to permethrin and methomyl throughout North America have lead to alternatives to chemical control such as resistant cultivars. Developed diamondback moth-resistant cabbage cultivars include those with glossy leaves and genotypes with normal waxy bloom. Resistant glossy varieties are only effective against 1st instar larvae, suggesting that the mechanism of resistance lies with the rejection by 1st instars, resulting in a high net movement and less feeding. This behavioral difference may lead to increased rates of larval starvation and desiccation on glossy varieties (Eigenbrode, 1990).
Sole dependence on chemical controls can lead to insecticide resistance in diamondback moth (Liu, et al., 1981). The diamondback moth has been able to develop resistance to many insecticides.
During peak diamondback moth seasons, controlling larvae alone does not immediately provide economic benefit because of the high egglaying capacity of adults. Additional sprays to control diamondback moth adults are needed during these periods. Our laboratory assays suggest that applications of mevinphos (Phosdrin) or naled (Dibrom) would be appropriate insecticides against adults. Because many adults are on plants adjacent to the field, application to the crop one or two hours after sunset would be very effective. Growers should conserve the use of these insecticides to reduce the chances of selecting resistant populations of diamondback moth.
There are no listings for Phosdrin and Dibrom as of April 2007.
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Danthanarayana, W. 1986. Chapter 7: Lunar Periodicity of Insect Flight and Migration. In: Insect Flight: Dispersal and Migration. Springer-Verlag Berlin Heidelberg.
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SEP/1991 revised MAY/1992.