October 2006. Diamond back moth in watercress. What is the best chemical(s) for control or management of DBM, Plutella xylostella, on watercress including the withholding period, questions asked by Cook Islands.
DBM poses a difficult problem on watercress. Most control strategies (be they biological or chemical or cultural) have been developed for field crops of crucifers and may not be appropriate for watercress.
It is difficult to make any suggestions/recommendations without more detail of the particular situation. However, many broad-spectrum insecticides (particularly pyrethroids) are extremely toxic to freshwater invertebrates and fish and should not be applied to watercress crops in waterways. The results will be disastrous!
A series of timed and sequential submersion of the watercress crop using weighted wire mess, starting from the upstream ‘crop’, on a weekly/fortnightly basis, would disrupt the pest’s population dynamics. This cultural method works this way:
Apart from eggs, caterpillars should either drown, get swept away by water currents or be eaten by fish or other aquatic organisms when they dislodge after submersion. The effectiveness of this will depend on:
- Strength of current (for adults or larvae to be carried away;
- Population and diet of resident fish and other organisms in the water. If they have been killed by pesticide than there will be no fish to eat the caterpillars as they dislodge;
- Length of time of submersion of the crop (try couple of hours): the longer the submersion the better
- Start the submersion beginning with clumps at the uppermost part of the stream/drain.
Drowning of DBM larvae was one of the more effective methods that one member had used for management of DBM in cabbage and cauliflower. The effect of spraying water at very high volumes – 1000 litres per hectare – effectively drowned the larvae (methods provided). The trick was in getting the timing right, i.e., when the majority of the population were early instars.
A system has been described for cabbage, which essentially aims at preventing the moth from spotting the cabbage. Cabbage plots, 3 x 3 m, are surrounded by maize. In India, Indian mustard has been used to attract DBM (one row in six rows of cabbage). It is quite an effective trap crop. Spraying is only required on the mustard.
As for chemicals, Bt was said to give excellent results, applied weekly, as necessary. However, The use of 1 product for control of DBM is not sustainable because DBM will inevitably become resistant to it, including resistant to Bt products.
The key reliance on insecticide applications for control of DBM must include a resistance management strategy, which is generally recognized as a rotation of different insecticide classes over time. If only Bt is used, then rotate between Btk and Bta.
In New Zealand, the rotation strategy is based on rotating spinosad (Success) and indoxacarb (Steward), but also the rotation of Btk and Bta products is recommended. Products that have a mix of Btk and Bta may be rotated with Bta products.
The rotation of these products must be over at least 1 generation of DBM. For example, in the tropics they could be rotated every month, but in cooler climates perhaps every 2 months or more.
However, another important aspect of resistance management is that the rotation of different insecticide classes should be on an area-wide basis.
The important point is to periodically change the class of insecticide used.