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Diamondback Moth (Platella zylostella) adults are the smallest of the brassica lepidopteran pests 1/2 inch, light brown with a yellow diamond-shaped marking, and rest with their wings folded together like a tent. Caterpillars reach 3/4 inch in length, are light green, and are segmented and pointed at both ends. When disturbed they wiggle vigorously and may drop off the plant on a string of silk. Feeding causes small, round holes and tends to be spread across the foliage rather than concentrated in the head. Scout fields by checking leaves underside on 25 plants across the field.
The larvae attack a wide range of cole crops including: cabbage, cauliflower, rape, kale, turnip, and brussels sprouts. In the Northeast, the diamondback moth is a sporadic pest, with four to six generations a year depending on locality.
Diamondback moth attacks only plants in the family Cruciferae. Virtually all cruciferous vegetable crops are eaten, including broccoli, Brussels sprouts, cabbage, Chinese cabbage, cauliflower, collard, kale, kohlrabi, mustard, radish, turnip, and watercress. Not all are equally preferred, however, and collard will usually be chosen by ovipositing moths relative to cabbage. Several cruciferous weeds are important hosts, especially early in the season before cultivated crops are available.
Diamondback moth eggs are oval and flattened, and measure 0.44 mm long and 0.26 mm wide. Eggs are yellow or pale green in color, and are deposited singly or in small groups of two to eight eggs in depressions on the surface of foliage, or occasionally on other plant parts. Females may deposit 250 to 300 eggs but average total egg production is probably 150 eggs. Development time averages 5.6 days.
Life Cycle: Larvae mature in 10 to 14 days and spin a loose cocoon on leaves or stems for pupation. Moths oviposit their tiny, round eggs singly on the undersides of leaves; eggs are difficult to find. Although they may occur all year round, especially in coastal areas, diamondback moths are often abundant in spring and early summer with populations increasing again in the fall.
Interestingly, this is one of the few Lepidoptera that overwinters as an adult moth. After mating in early spring the females deposit small, almost round, yellowish-white eggs singly or in small groups on both sides of leaves of host plants. Larvae hatch after a few days and begin feeding just under the surface of the leaf tissue. After several days of feeding, mid-instar larvae exit the leaf and feed on the leaf surface.
Larvae cease to feed when the temperature drops below 50 F. As with other insects development through all life stages is delayed during cool temperatures. In contrast, populations can increase dramatically when temperatures rise above 80 F. The life cycle may be as long as 50 days at low temperatures and as short as 15 days at high temperatures. Mature late-instar larvae are pale green and about 1/3 of an inch long.
Damage : The diamondback moth larva can damage cruciferous plants by feeding and mining. Upon hatching, the first instar larva burrows into the cruciferous leaf and begins mining between the upper and lower leaf surfaces. The leaf mining injury to the plant normally is negligible unless extremely high populations occur. The later instars feed on the leaf surface, generally on the underside of leaves, making small, irregular holes while leaving the upper leaf epidermis intact and giving a window-like appearance to the feeding site.
Diamondback moth is the most serious pest of cabbage and stocks. Infestations are most serious when they damage the crowns or growing points of young plants. This injury can severely stunt growth. Diamondback larvae may affect yield or flower production if feeding occurs in the heart leaves prior to heading. However, in cabbage, once the plant has headed, feeding is usually found on the outer frame leaves, which are discarded at harvest. Thus, larger larval populations are necessary at this stage to cause plant damage and necessitate control. In stocks, larval feeding up in the floral stalk may exacerbate botrytis bloom rot, and the presence of larvae presents a contamination problem for the fresh cut flower market.
Monitoring Check fields during the seedling stage, at thinning, and just before heading. Also, record diamondback larvae numbers when you make your twice-weekly samples for other caterpillar pests. In cabbage fields, regularly monitor wrapper leaves for damage after heading. Adult moths frequently migrate from fields being harvested or disced under, so carefully check border rows if populations were high in adjacent fields. No treatment levels have been developed for diamondback moth in California; however, treatment may be required if significant injury to growing points is occurring.
Control: Natural enemies and insecticides applied to control other pests keep the diamondback moth under satisfactory control in most fields in California, but keep records of diamondback moth as you monitor for other caterpillars.
Biological Control Natural enemies often effectively control diamondback moth in California. In southern California, the ichneumonid wasp, Diadegma insularis, has been identified as the most common parasite. Trichogramma pretiosum may also attack diamondback eggs. Various predators such as ground beetles, true bugs, syrphid fly larvae, and spiders can be important factors in controlling populations. Microbial diseases are not known to be a significant mortality factor.
Organically Acceptable Methods Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable management tools.
Consulting a Pest Control professional should be considered if widespread clothes moth infestations develop.
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