Time to start monitoring for diamondback moth in Asian Brassica vegetables

Nancy Endersby & Peter Ridland, Institute for Horticultural Development - Knoxfield

It's time to watch out for caterpillars in Brassica vegetable crops. Caterpillars (larvae) of the diamondback moth (DBM), Plutella xylostella, can inflict serious damage on Asian vegetables such as Chinese cabbage during the warmer months of the year. Insects develop faster in warm weather than in cool weather. Generations start to overlap during summer and DBM numbers can build up very rapidly. From late spring to late autumn it is important to check for presence of DBM in crops in southern Australia. Regular crop monitoring and record keeping are the cornerstones of successful Integrated Pest Management (IPM) programs.

Reliance on chemicals as a control measure has resulted in the development of resistance to many insecticides by populations of DBM throughout the world. Resistance to synthetic pyrethroid insecticides has been detected in populations of diamondback moth in all Australian states. Since 1993, Brassica growers in Victoria have had difficulty in controlling the caterpillars of diamondback moth and some have experienced insecticide control failures.

The life cycle of diamondback moth was described in detail in Issue 13, October 1998 of this newsletter. Moths lay eggs which develop into caterpillars. Caterpillars feed until they reach their maximum size, then spin a cocoon, pupate and emerge as a moth.

National DBM Project

We are now into the third year of our National Horticultural Research and Development Corporation DBM project. The project aims to improve control of DBM with existing insecticides, to limit development of resistance to new insecticide families and to develop cost-effective alternative control strategies.

Key project recommendations for management of DBM in 1999 / 2000

• Use pheromone traps to monitor the build-up of moths, particularly early in the season. An increase in moth numbers should be a prompt to monitor the crops and look for eggs and caterpillars. Traps may be purchased from Dunluce International Pty Ltd (02) 9983 1776 and some chemical resellers.
• Crop hygiene. Start off with healthy transplants which are free of caterpillars and plough in crop debris as soon as possible after harvest.
• Monitor the crop once per week by inspecting randomly selected plants throughout the paddock. Record pest numbers and their development stages. Use these counts to judge whether a spray is required and to check whether control tactics applied previously have been successful. The more plants you look at, the more accurate will be the assessment of what is happening in the crop.
• Use Bacillus thuringiensis (Bt) to allow DBM a break from exposure to synthetic insecticides and to protect natural enemies of pests. Bt is a bacterium that is formulated into a biological insecticide. Bt products registered for use on Brassica crops kill only caterpillars. Ensure good spray coverage of the plant, as the pest must eat Bt for it to be effective. Do not expect instant knockdown. Apply Bt after irrigation, not before. Best results are obtained on small caterpillars (less than 5 mm long).
• Follow AIRAC's insecticide resistance management strategy. Project staff around Australia have worked closely with AIRAC (AVCARE's Insecticide Resistance Action Committee) to develop an effective Insecticide Resistance Management Strategy for the new chemical families available. A central feature of this strategy is separation of the calendar year into two windows, with the use of each new chemical being restricted to one window. The strategy aims to delay the development of resistance to new insecticide groups.

AIRAC DBM Insecticide Resistance Management Strategy for New South Wales, South Australia, Tasmania and Victoria 1999/2000
September to January	February to August
Bt Secure®, Success® Other registered insecticides (except Regent®)	Bt Regent® Other registered insecticides (except Secure® & Success®)
Secure® and Success® may be used throughout September to January. Regent® may be used throughout February to August. Labels of the new products limit the number of applications per crop. Other insecticides registered for DBM may be used at any time of the year. It is important to monitor crops regularly for DBM. Do not use mixtures of insecticides for controlling DBM. Use of the biological insecticide, Bt, in the early stages of crop development is encouraged at all times of the year. Good crop hygiene such as use of clean seedlings will reduce your DBM problems.

Project research - Where do the moths come from?

We are trying to answer some important questions such as:

• Do moths move into the vegetable growing areas in spring or do they emerge from old crops?
• Are the moths on canola and weeds resistant to insecticides?

This spring we have been sampling Brassica crops (vegetable, forage and canola) and weeds in key areas in the state to assess levels of DBM activity and potential sources of moths.

In Victoria there are some extensive canola crops to the north west of the vegetable growing district of Werribee South. Preliminary results suggest that insecticide susceptible moths are moving from the canola to the vegetable crops. Such movement would be most helpful for delaying development of resistance to the new insecticides. We are doing further testing in spring 1999 to assess permethrin resistance status of diamondback moth from a number of crops throughout the Werribee area and in nearby canola crops.

Growers in other states contact:

Queensland
John Duff (Gatton), Tel. (07) 5462 2222
Western Australia
Françoise Berlandier (South Perth), Tel. (08) 9368 3249