Diamondback moth (DBM) -
What's going on in the 1998/99 season?

Nancy Endersby and Peter Ridland, IHD Knoxfield, Tel. (03) 9210 9222

Caterpillars of diamondback moth, Plutella xylostella, only feed on members of the family Cruciferae and are a particular problem on Asian Brassica vegetables such as Chinese cabbage, Chinese broccoli (kailaan), Chinese flowering cabbage (choy sum) , Chinese chard (pak choy), Shanghai Chinese chard (Shanghai pak choy), Baby Chinese chard and Mustard green (kai choy). They also infest Brussels sprouts, cabbage, cauliflower and broccoli.

An Insecticide Resistance Management Strategy for the new chemical families has been developed with AIRAC (AVCARE Insecticide Resistance Action Committee). A central feature of the strategy is to divide the calendar year into two production windows. Growers in most states have two lists of insecticides to choose from. One for September - January and a different list for February - August. A four-window strategy has been developed for Queensland growers. Use only one type of chemical in one window. This strategy will be available in late 1998 and will be published in this newsletter. A colour-illustrated Integrated Pest Management (IPM) Manual is being prepared. Management of DBM and most crop pests is best approached by integrating a wide range of pest management tactics to prevent pests from reaching damaging levels in crops. Regular crop monitoring and record keeping are the cornerstones of successful IPM programs.

Key IPM recommendations for management of DBM in 1998/ 99 Use pheromone traps to monitor the build-up of moths, particularly early in the season (spring). An increase in moth numbers should be a prompt to monitor the crops and look for eggs and larvae. Pheromone traps can be bought from some chemical resellers or from Dunluce International Pty Ltd (02) 9983 1776. Use Bacillus thuringiensis (Bt) to allow DBM a break from exposure to synthetic insecticides and to protect existing natural enemies. Bt is a bacterium that is formulated into a biological insecticide. Bt products registered for use on brassica crops kill only caterpillars and are very valuable in managing DBM. Ensure good spray coverage of the plant, as the pest must eat Bt for it to be effective. Do not expect instant results as caterpillars take 2-3 days to die but they do not feed after eating Bt. Apply Bt after irrigation, not before. Best results are obtained on small caterpillars (less than 5 mm long). Crop hygiene. Start off with clean, healthy transplants, choose less susceptible you have found to be varieties, and ploughin crop debris as soon as possible after harvest. Ask your supplier to use Bt on your transplants. A summer crop break. Plan with your neighbours (within several km) to have a break from crucifer production of 2 to 3 weeks in early to mid summer to starve out DBM. Use improved spray application and strategic application of insecticides according to the two-window strategy (available soon). Monitor the crop and spray only when necessary.

Life cycle Eggs of DBM are very small and difficult to see. At times of high pest pressure, they are found in clumps on both sides of the leaves and on stems of the plant. They are pale yellow, oval shaped and less than 1 mm long. When the egg hatches, the tiny caterpillar mines into the leaf and feeds on the internal leaf tissue. Subsequent growth stages of the caterpillar feed on the outside of the leaf making numerous small holes, like hail damage, or leaving the upper leaf surface intact as a characteristic opaque "feeding window". The final growth stage causes the most damage and is very difficult to control. When fully grown, the caterpillar spins a protective cocoon and pupates inside. It emerges as a moth (12 mm long, brown with a white diamond pattern along its back). The moth lays eggs and begins the cycle again. The life cycle of insects is dependent on temperature. Insects develop faster in warm weather than in cool weather. During winter, the pest develops slowly and causes little damage. As the temperature increases in spring and summer, the moth goes through its life cycle more quickly and pest numbers build up rapidly. The message for growers is to be very cautious whenever there is a spell of hot weather even in spring or early autumn.

Eggs     Larvae     Pupa     Moth

Crop monitoring Detailed crop monitoring studies in South Australia and Victoria are leading to the development of simple, grower-friendly sampling schemes. Regular monitoring enables growers to spray only when required and to check on the success (or failure) of previous treatments. Other benefits include: Detecting the build-up of pests well before economic damage occurs Making correct decisions on whether control measures are necessary Optimising the timing of spraying or other control measures Selecting the most appropriate control measure Identifying problem varieties and areas within crops How do I monitor? Inspect randomly selected plants at least weekly. Look at the whole plant, including both sides of leaves. Look for eggs on the stem and leaf stalks. Walk a zigzag or figure-8 pattern through the crop, starting at a different place each assessment and looking at plants at regular intervals. A 10 x magnification hand lens is useful. Record pest numbers and their development stages. Use these counts to judge whether a spray or other management tactic is required and to check whether control tactics applied previously have been successful. The more plants you look at, the more accurate will be your assessment of what is happening in the crop. Record your weekly egg and caterpillar counts. What use are insect records? Each week's records can be compared to see whether pests are becoming more or less abundant. The records will show whether a previous spray application had the desired effect. Long-term records will reveal whether some areas of a paddock consistently harbour more insects; for example, along edges. They will also indicate when pest pressure is greatest. Observations of beneficial insects may be useful in determining whether a spray is required or whether another control measure would be better. Keeping records may confirm suspicions that a particular variety always hosts a lot of pests or that stressed plants are more attractive to the pests. At times of high pest pressure it may be possible to avoid growing the more susceptible varieties.

To identify DBM, contact your local: Agriculture New South Wales Leigh James (Windsor) (02) 4577 0622 Queensland Department of Primary Industries John Duff (Gatton) (07) 5462 1122 South Australia Research and Development Industries Greg Baker (Adelaide) (08) 8303 9544 Tasmania Department of Primary Industries & Fisheries Lionel Hill (Devonport) (03) 6421 7601 Agriculture Western Australia: Françoise Berlandier (South Perth) (08) 9368 3333 NT Department of Primary Industries and Fisheries Graham Young (Berrimah) (08) 8999 2163 Victorian Department of Primary Industries Stephen Moore (Geelong) (03) 5226 4667 Subra (Bairnsdale) (03) 5152 0600 Murat Top (Tatura) (03) 5833 5222 Greg Hayes (Myrtleford) (03) 5731 1222