13
Rice Brown Planthopper
of macropterous BPH (1910/200hills) existed in the field on 21st standard week
along with high population caught in light trap (38715/week). (Table 3 In the chapter
“Ecology and Population Dynamics of BPH”). By 22nd standard week the macropterous
BPH population (both males and females) present in the field dropped to very low level
(271/200 hills),but the light trap catches of macropterous adults continued to be very
high(32710/week). The insects caught in the light trap could only be those which are
airborne and must be representative of those leaving the rice crop (Krishnaiah et al,
2006).
Crowding or nymphal density is not responsible for higher macroptery:
There are many reports mainly from tropical countries that crowding or higher nymphal
density is also a factor in determining the proportion of macropterous adults. As the
first instar nymphs emerging from the eggs laid by macropterous females settling on the
young crop or those which are emerging from the eggs laid by brachypterous females
in the later generations are bound to be in high numbers in a hill. Had the high nymphal
density been a factor that induces high macroptery then it would have certainly affected
the very continuation of BPH in the field even when food quality and temperatures are
favourable. Thus high nymphal density is not having the macroptery inducing effect
and continuous emergence of brachypterous adults is a key to perpetuation of BPH as a
damaging insect to rice crop. But in some of the studies under glass house conditions in
India high proportion of macropterous adults were observed at higher nymphal densities.
The main reason for this could be faster deterioration of food quality when more nymphs
were confined to potted plants. Uniform food source has to be maintained for all nymphal
densities as an experimental requirement. The plants exposed to high nymphal densities
tend to wilt or actually wilt even before those are replaced the next day.
Difference in period of high mating potential in macropterous and
brachypterous males and its significance:
Another important point is with regard to mating ability of males. Usually
brachypterous males are far more active than their macropterous counterparts
immediately after penultimate moult. But they become incompetent compared to
macropterous males after 4-5 days. Usually macropterous males are poor mates
immediately after emergence but become more effective mates after 4-5 days during
which period brachypterous males become incompetent (Novotný, 1994). This appears
