7
Rice Brown Planthopper
Distribution of insects on the whole is patchy or clumped. There will be very high
population at some points and very low populations just nearby areas. This is
statistically called “negative binomial distribution” (Kusmayadi et al.,1990).
Suitable and suggested sampling method for assessing BPH population in the field is
random sampling in quadrats of four adjacent hills. Number of such quadrats can vary
depending on stage of plants and required precision level. But practical experience
of the author and many other workers showed that random sampling on fixed single
hills at different stages is a better option. About 200 such hills fixed at random and
distributed almost throughout the area can give a reasonably good estimate of BPH
population in one acre field. However, for smaller plot sizes 20-25 fixed random hills
are sufficient. Economic threshold level for BPH has been suggested as 15-20 insects
per hill and for WBPH 20-25 insects per hill after 45-50 days after planting. The values
can be lowered to 10 insects per hill for BPH and 15 insects per hill for WBPH, if
damage occurs in early stages.
Morphoforms of Planthoppers
Adults of plant hoppers generally have two morphologically distinct forms. 1) Those
with long or fully developed wings or macropterous adults (Plate 1 Page 2) and 2)
those with short or half developed wings called brachypterous adults (Plate 2 Page 2).
Both the forms are present in males as well as in females in all the three hoppers viz.
BPH, WBPH and SBPH, but brachypterous males are rare in WBPH. Macropterous
adults are capable of flying over long distances, sometimes extending to hundreds
or few thousands of kilometers. These are capable of withstanding starvation during
the flight until they reach a suitable habitat, which is usually a freshly planted rice
crop. However, the macropterous adults can lay substantially low number of eggs
per female, usually in the range of 70-100. Thus, macropterous forms are evolved for
migration from fields or areas where conditions are not favorable to the favorable areas.
Majority of the nymphs emerging from the eggs laid by macropterous adults in the new
habitat become brachypterous or short winged forms. These are robust and capable
of laying 300-400 eggs/female. Majority of the nymphs from these eggs also become
brachypterous adults enabling very rapid increase in population. At the time of first
settling of macropterous forms, the insects are very sparsely distributed, usually 1-2
insects/10-100 hills. Within 2-3 generations, the populations raise to 40-100 insects/
hill which is far above the economic threshold level of 10-20 insects/hill and can cause
