Distribution of Darna trima (Moore) and D. bradleyi Holloway larvae (Lepidoptera: Limacodidae) in oil palm canopy, in a single species and a double species infestations

In single species infestation by Darna trima, the larval density was highest in the middle of the canopy (front 20), with the relationship between density of larvae and frond number described by y=33.8 + 19.2x -0.48x**2 (r**2=0.75, P less than 0.001). The larval densities of the distal and middle sections of a frond were more than twice that of the proximal section (all sections had equal number of pinnae). In infestation predominantly by D.trima and D. bradleyi (double species infestation), the larval density of D. trima was related to front number by the equation, y = -27 + 194x -12.8x**2 - 0.21x**3 (r**2=0.96, P less than 0.001) which showed a peak in density at frond 9. There was no significant difference between the number of larvae on the left (446 plus minus 70) and right (422 plus minus 110) side of the fronds. However, the distal and middle sections had respectively five times and eight times more larvae than the proximal section. For D. bradleyi, the relationship between larval density and frond number is described by y = 24.6 - 1.61x + 0.22x**2 (r**2=0.80, P less than 0.001). No significant difference was recorded between the right (101 plus minus 65) and left (46 plus minus 54) side of the frond, although significantly more larvae (5 times more) were found on the middle section of the frond than the proximal section. The results seem to suggest that for a reasonable pre- or post- treatment census of D. trima, fronds 15-25 may be used in the single species infestation, while fronds 7 - 11 may be sampled in a double species infestation. This takes into consideration other factors such as age of palm, level of damage, larval density level and number of fronds on the palms etc. Furthermore, the number of larvae on one side of a frond can be used to estimate the total number of larvae present per frond, although the larval density may vary along the length of the frond. We recommend that studies on distribution of all the major leaf-eating pests on oil palm be carried out for successful implementation of IPM of these pests in oil palm in this region.