August 2003. Watercress is a main source of income for growers a few kms outside Honiara. Plants are grown in a slow moving stream. The foliage is being attacked by Diamond back moth, and sprays of ICON, lambda-cyhalothrin (3 times a week!) are not now controlling them. The problem started in 1998 and has got progressively worse. The growers say they have tried other chemicals, but they are no longer effective. Some people would like to remove the plants from the rafts and start again, but not all agree to do this. The caterpillars in the photos are about 8 mm long, and the body of the adults about the same length.
The pest was identified as Plutella xylostella, Diamond back moth. It was also mentioned that the insect has been a major problem on crucifers in Jamaica and indeed, worldwide. The insecticide, Bacillus thuringiensis (known under trade names: Xentari, Agree, Thuricide) has worked very well against the pest in Jamaica. Insect growth regulators (e.g. Match – flufenoxuron) have also proved useful. These insecticides will help conserve natural enemies of the pest. That is critical to sustainable management of Diamond back moth as it develops resistance to pesticides so rapidly.
In work by the University of the West Indies, tobacco plants intercropped with crucifers were also shown to repel moths. If used on watercress, they would need to be put in pots and placed on the rafts. In cabbages, one plant is placed in the centre of 15 x 15 rows of cabbages.
Work under the IPM-CRSP in Jamaica has also revealed the efficacy of new compounds against lepidoptera larvae. One is Spinoace (trade name of the product in Japan), a product from Saccharopolyspora spinosa and, perhaps, better known as Spinosad and the trade name, Success-Editor. Insect-proof cages are another alternative.
In New Zealand a lure is available: Graeme Clare at HortResearch, Auckland; email: [email protected]. However, it is thought to have limited use in an IPM program, because crops need to be regularly scouted for other pests and, when DBM arrives, very quickly there are overlapping generations, so moth counts mean little.
However, when first planting brassicas in a new season or area, pheromone traps can forewarn of larval infestations by 1-2 weeks.
Information is available in the Proceedings of New Zealand Plant Protection 56: 180-184 (2003) on: www.hortnet.co.nz/publications/nzpps.
It was pointed out that Plutella maculipennis (Curtis) is now synonymous with Plutella xylostella (Linnaeus).
Other photos are at: images.google.com/images