September 2008. A story about the use of nets over beds containing the crucifer, petsai (Brassica pekinensis) in southern Mindanao, Philippines. The commercial operation is at 1500 feet above sea level, with some 4 ha of river valley on layered sandy to heavy clay soils and a climate varyiing from long dry to long wet periods. Last year was hot and favoured diamond back moth – this year is cold and wet and has prevented its occurrence.
Inorganic and organic pesticides have proved ineffective (they are also expensive), so beds were netted with 30 m x 1 m, 50 micron nylon moquito nets. In conjunction with locally available pesticide treatments these were successful against adults, but did not stop early infestation, even though all the seedlings were grown under nets and checked before planting out. Later, it was discovered that the adults were laying eggs on the nets over the outer plants or directly on the plants through the nets. This was supported by the observation that it was always the outer rows that were attacked first. The attacks were so bad that they stopped the production of petsai.
Later, still (this year), a different planting system was introduced: a 1 metre space separates the nets from the plants and overhead irrigation has been installed. Further tests are planned to determined how far the first instars can crawl or absail, and how far the adults can sense the plants to start laying on the nets.
Members were intrigued by the story. In India, Indian mustard is used to attract diamond backed moth. So, Indian mustard could be planted around the net houses. The mustard plants must be sprayed or removed when infestation is high.
In a later communication concerning the use of nets, the farmer asked about the ability of the 1st instars to crawl and about the ability of the adult to sense its food crop. In response, an expert from New Zealand said that larvae were able to crawl through mosquito netting, but generally does not crawl far – its main movement is by absailing on silken threads; the female is stimulated to oviposit by smell or taste, and will lay on artificial surfaces if stimulated to do so or ‘desperate’ to lay before it dies.
There was also a recommendation to examine whether the right pesticide application techniques were used to control DBM. DBM eggs are normally laid on the underside leaves and the 1st instar burrow into the leaf tissues (too small to see or a “sort of punctured-holes” is seen. The 2nd instar are more visible to the naked eye. In general, the larvae stays on the underside of the leaves most of the time (on crucifers plants, at least).
If Bt is used, the spray should be aimed at the underside of the leaves. Bt acts as a stomach poison and there needs to be good coverage on the leaves to have the best result. Or else, a systemic insecticide that has translaminar activity could be used. With this, the poison would reach the pest even if the spray coverage is incomplete.
Another point is the pH of the water and its microbial content; these may influence the viability of Bt or the active ingredients in the pesticide.
In addition to these comments on the use of synthetic pesticides, a member wrote concerning the use of fungal biopesticides for the last 5 years to manage DBM for crucifers in Indian. There are now some highly effective oil based formulation of Beauveria bassiana; these need to be used only 3-4 times per crop at 2 ml/litre (1.5 litre/ha/spray). The efficacy of this product has been extensively tested on India crucifers, particularly on cabbage/cauliflower, and found to on par with BT and Indoxicarb in terms of managing DBM populations. The only limitation is that it has to be sprayed at the onset of head formation. Mortality rate is low at the beginning but gradually increases and keeps the population below threshold levels. There has been good feed back from farmers. The product has been registered and made is available in commercial form; it is called MYCO-JAAL. Of late, the product is exported to Taiwan, Malyasia and Nepal.
Another member warned that DBM has become resistant to every ‘toxin’ used against it, including Bt sprays! To minimize resistance rotate different classes of insecticides. With the Bt sprays, rotate a Bt k (kurstaki) strain with a Bt a (aizawai) strain. Change insecticide classes every month or two, to target different generations of DBM – a proven resistance management strategy.