Kyushu Plant Protection Research

Seasonal changes in numbers of the melon fly, Dacus cucurbitae COQUILLETT, have been investigated by counting numbers of males caught in Cue-lure bait traps on Kikai Is., Kagoshima Pref. since 1974 when the species had first invaded the island. Total numbers of annual catches rapidly increased in 1976 and 1977, and such high density as over 10, 000 males per 50 traps in 1977 was considered to be related to richness of Diplocyclos palmatus C. JEFFREY (Cucurbitaceae). In general, there was a tendency for the melon fly population to increase in autumn and to decrease remarkably in winter. Relative abundance of the melon fly was also investigated in 1977 at several different habitats on the island. The melon fly was most abundant in isolated groves, and it was low in number inside villages. These differences in the relative abundance cannot be ignored for estimating and controlling the population density of the species in the future.

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The occurrence of the broad mite, Polyphagotarsonemus latus (BANKS), was observed first in the fall crop of potatoes at several places in Nagasaki in 1975. The mites were supposed to migrate from other host plants adjacent to potato fields. The propagation of mites on the potato variety, Shimabara, was poorer than on the varieties, Dejima and Tachibana. A greenhouse experiment indicated that the growth of mite population was proportional to the initial number of mites placed on the potatoes, 2, 5 and 10 mites per plant, up to 25 days. However, the number of mites including adults and nymphs per first small leaf reached about 150 in all plots 30 days after the initial infestation, and soon after the mites completely withered up the potatoes. Spraying of chinomethionat (Morestan) WP at the rate of 750g AI/ha gave a satisfactory control of mites in pot and field experiments, but acephate (Ortran) WP was ineffective.

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Influences of low temperature on the motility of second stage larvae of Meloidogyne incognita reproduced on sweet potato variety "Norin No. 1" were examined. The motility was assessed by the downward movement of larvae through a cotton-wool filter in a glass vial. Among three types of larvae examined, those hatched wthin 48 hr at 25°C from egg-sacs collected in August (EL-8) indicated the highest motility. Those collected from soil by sieving technique in August (SL-8) came next, and those collected from soil in February (SL-2) showed the lowest motility. More than 50% of individuals moved in 15 days at 8-C in EL-8, 10-12°C in SL-8, and 12°C in SL-2. The differences in the motility between EL-8 and SL-8 suggest that after hatching certain physiological changes which lower the motility may occur in EL-8. And the differences in the motility between SL-8 and SL-2 suggest that low temperatures in late autumn and winter may condition low motility of SL-2.

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The effect of different humidities on the development of Aniblyseius deleoni MUMA et DENMARK was examined. Most of the eggs were unable to hatch at relative humidities lower than 56% at 20°C. The high mortality of immature stages was observed at 70-75% R. H. at 25°C and their developmental period was longer than that at 85-90% R. H. The effect of humidity on the ability of predators to regulate the mite populations, Panonychus citri MCGREGOR, was examined at 55-60, 65-75 and 85-90% R. H. at 25°C in the growth cabinet. When the initial ratio of predators to preys was 1 : 20, the predator suppressed the prey populations within 21 days at 85-90% R. H., but no suppression was observed at lower humidities than 65-75% R. H. The increasing rate of the predator population at 65-75% R. H. was slower than that at 85-90% R. H.

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