12
Rice Brown Planthopper
instar are also triggered. This facilitates all those first instar nymphs feeding on
nutritively rich plant sap to become brachypterous adults with very high reproductive
ability. They continue to give raise to brachypterous progeny until flowering. During
all this time both quality food and favourable temperature exist. Once senescence starts
in the crop towards grain hardening stage, the quality of food will adversely affect the
insect although temperatures continue to be not adverse. The movement the first instar
nymphs emerging at the beginning of grain hardening stage taste the poor quality food,
high in sugars and poor in protein and free aminoacids, then JH III production process
is inhibited and JH III titre will be low throughout the life of all those insects. So, all of
them invariably become macropterous individuals. Thus they can start migration even
when the crop is still standing. In this way, the pre signaling with low hormonal release
and subsequent low hormonal regulation enable the insect to prepare itself much earlier
than the actual adversity dawns upon BPH towards senescence of rice crop.
During rabi or second season, usually cold climate exists at the beginning of the season.
So macropterous insects after settling, lay usually fewer eggs than their counterparts
during kharif season or rainy season. So their initial build-up itself will be very poor.
By the time temperatures rise to optimum level for BPH the crop growth is advanced
and then only the insects start multiplying faster. Usually duration of the varieties
grown during rabi or summer season is shorter than the kharif grown varieties. So the
net time available for the insect to multiply to damaging levels is less. That is why
in most of the areas where double crop is a common practice BPH damage is less
during rabi compared to kharif even in the varieties of almost the same duration or
even when the same variety suitable for both the seasons is grown in a particular area.
Secondly towards the end of the season, temperatures tend to be higher much before
the flowering starts in the crop. These higher temperatures are unfavourable for BPH.
During rabi season high temperatures act as a triggering mechanism for lower JH III
production. So, high proportion of nymphs tend to become macropterous adults even if
the food is still favourable for growth. Thus pre-signalling of lower JH III production
by poor quality food and adverse temperatures enable BPH to move always away from
places of forthcoming adverse conditions to favourable places.
Practical field observations of the author and many rice entomologists very clearly
reveal that 15 days before the harvest of rice crop the total population present in
the field are macropterous adults both males and females. The field observations
during kharif 2001 at Maruteru clearly showed this phenomenon. High population
