Vergara - 1976 - Physiological and morphological adaptability of ri

378 CLItvLATE AND RICE
1965. All but three of the outbreaks occurred when rainfall exceeded the average
89 cm. One of the exceptions is explained by rain having anived very late in
April, and in the other two outbreaks, rainfall was nearly average. The reason
for not having outbreaks in 2 years when rainfall exceeded the average is unknown.
Miyashita (1963) reviewed records of outbreaks of various insect pests. and
found that outbreaks of M. separam in 1949 covering the whole of Japan except
Kyushu were related to low temperature and heavy precipitation during the
late spring and summer.
Koyama (1964) reviewed records of outbreaks in Akita Pref. Japan. from
1912 to 1963. and said that before 1955. twutbreaks tvere sporadic. spatially‘
restricted, and occurred a month. or one and a half months. after a flood.
However. after 1955, outbreaks (recurred more or less every y'ear. and simultaneously
in many localities. He suggested that the tenderness of the rice plant.
particularly at younger stages, induced by heavy application of nitrogen fertilizer.
favored survival of young larvae of the first gtillfiftlllOn in June and July
when the armynwonn is usually extremely sparse. 1t is noteworthy that recent
outbreaks often include ilifviliimnr: Iorevi Duponchel, which is a species closely
related to .11. separata. but has been considered a non-outbreak type.
The two species tend to la_v eggs on withered plant stems or leaves. and it is
possible that moths prefer to gather on willed plants after flood or drought.
The annytviann is a clear case of "phase-variation“ in color and behavior
(Iwao. 1962). Phase variation is considered to develop in species which inhabit
unstable vegetation. and mass migration seems to have adaptive value in C011-
fronting a sudden change ofhabitat. The real process of outbreak ofrtl. separara
appears to be related to long-distance migration. as will be mentioned latcr.
Regarding the effects of rainfall on the population dynamics of other insects.
high rainfall early in the year. light rainy periods. and cloudy’ skies have often
been identified as the factors causing outbreaks of the rice gall midge, Pachydiplosis
oiyzae (Wood-Mason). Prakasa Rao. Rao, and Israel (1971) noted that
in India. the onset of the midge was delayed in years when pre-monsoon rains
were below 250 mm in May and June. Apparently early rains cause weeds. the
alternate hosts of the midge, to grow more than usual. By the time the midges
move on to rice, their population is already high. and a serious infestation
results. Rainy, cloudy weather during the early crop period encourages suntival
and growth of the population. Populations decrease with low humidity. The
gall midge requires high humidity for survival and multiplication. and thus is
not a serious pest in the dry season (Fernando. 1971; Hidaka and Vungsilabutr.
1971'. Kovitvadhi and Leaumsang. 1971; Prakasa Rao. 1972; Vungsilabutr et
a1, 1972; Hidaka. 1973'. and Prakasa Rao. 1974).
Rainfall is thought to influence stern borer populations as well. Ngoan (1971)
reported that. in South Vietnam. (“kilo sirppressalis was more serious on the wet
season crop. and Tryporjvza incerrzilas on the dry season crop. In India low rainfall
in the kharif season (usually a wet season) is thought to favor outbreaks of