Chemical control of Helopeltis theobromae Mill and crop loss assessment in cocoa [Theobroma cacao]

In Peninsular Malaysia, chemical control of H. theobromae on cocoa is generally needed and gamma-HCH (lindone) at 250 g A,I/ha (repeated at 2 weeks) is conventional. The following three trials were done. In the first trial, two different strategies of response to census (with an untreated control) were compared in single estate-demarcated block of size 6 to 44 ha. Census for fresh damage to cherelles, pods, and sheets, was done monthly on 1 tree in 10, 1 row in 10. Treatments (strategies) did not commence simultaneously but evolved with perceived opportunity. In the plot response each plot of 10 x 10 entred on the census tree was sprayed conventionally whenever the tree had fresh damage in the 'threshold response' the entire block was sprayed when 25 % of the plots had fresh damage. In the plot response treatment in 1984, 1985 and 1986, a total of from 104-155 % of the area of the block was treated. In the threshold response treatment, the block was sprayed twice in both 1985 and 1986. In both strategies, the proportion with fresh damage was less that in the untreated block, whilst pods produced from February 85 and January 87 was 296 % of the untreated block in 'response' and 202 % in "threshold". In the second trial, unreplicated cocoa areas of 27-141 ha, of 3 to 6 year old cocoa on uneven terrain, were sprayed with gamma-HCH from a fixed-wing air craft. In 1985, comparisons were made of 250 g. A.I./ha (two successive sprays) at 100 and 300 um droplets, in 1986, of 250 with 500 g A.I./ha, both at 300 um droplets. All sprays were effective, with no adverse side-effects. There was reduction of nuisance mosquitoes. In the final trial, eighteen insecticides including synthetic pyrethroids, carbomates, organophosphates and insect growth regulators were compared for kill in replicated field trials. Lambda-cyhalothri (7.5 g A.I./ha), and dioxacarb (120 g A.I./ha), gave better kill than gamma-HCH; others were less effective.