Kyushu Plant Protection Research

A gradual but significant change in biotype property of the yearly immigrants of the brown planthopper (BPH) was observed during the period from 1987 to 1991 in Japan. Saikai-184, a breeding line of rice with the Bphl gene, was resistant to BPH immigrants in 1987, but became susceptible in 1990. Honeydew measurements with BCG-filter paper and parafilm sachet revealed that the BPH females immigrating in 1990 and 1991 excreted as much honeydew on Saikai-184 as on the susceptible variety Reihou. Also, the 1987 immigrants excreted little honeydew on IR26 with the Bphl gene, but the 1990-immigrants excreted considerable amounts of honeydew on it. The percentage of individual females which excreted more than 10 mg of honeydew per day was only 10% in the 1987-immigrants, whereas it was increased up to 30-40% in the 1990-immigrants. Both the 1987- and 1990-immigrants had little ability to feed on IR42 with the bph2 gene. These findings indicate that the BPH immigrants into Japan are changing from biotype 1 to biotype 2. Similar biotype shifts have been reported correspondingly in south China and north Vietnam. This may indicate that south China and north Vietnam are the possible source areas for the overseas immigrants of BPH into Japan.

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The effect of pipunculid parasitism on the population dynamics of the green rice leafhopper, Nephotettix virescens was studied in asynchronous rice planting areas of Central Java and Bali, Indonesia. The parasitism rate of N. virescens adults was higher in migrant-producing, 1st and 2nd generations in paddy fields than in the other generations which were mainly composed of immigrants from outside. These included populations obtained in seed beds, paddy fields of 1-4 weeks after transplanting and ratoon fields of 1-4 weeks after harvest. Since the parasitism occurs mainly in the nymphal stages, the above results indicate a suppression of migratory activities of parasitized N. virescens. The parasitism rate of 1st generation adults was related positively with the adult density and negatively with the percentage of mature females among parasite-free females. The effects of a higher emigration rate of healthy adults on the parasitism rate of resident populations were analyzed with mathematical models. The results showed that the role of pipunculid parasitism as a factor stabilizing or destabilizing population fluctuations of N. virescens is quite limited.

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Populations consisting of Meloidogyne incognita race 1, 2, 3, M. javanica or M. hapla collected in Kyushu were inoculated to the North Carolina standard hosts and sweet potato cultivars, Norin 1 and Norin 2 with a standardized inoculation method adding 1, 000 second-stage juveniles to each potted differential host plant. Meloidogyne spp. and races were distinguishable by the host or nonhost status of the differential host plants. When artificially mixed M. incognita race 1 and M. javanica were inoculated to tobacco (NC95) and pepper, suppressive interference of the latter against the former seemed to occur, but a linear fomula could be applied to extrapolate the ratio of species in a naturally occurring mixed populations previously estimated morphologically. This method may be useful in estimating the ratio of Meloidogyne spp. present in mixed popula-tions, but further work remains to be done on other Meloidogyne spp. and races.

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The Pratylenchus vulnus infested soil was collected from the field and mixed evenly to minimize the variation in nematode density. The mixed soil was then put in 60 plant pots (about 5.3 liter). Three experimental plots were set, consisting of 20 pots which were buried at a distance of 20 cm in the row. Marigolds var. African tall and strawberries var. Toyonoka were individually planted on 10 July 1991 in the pots of each plot while the other 20 pots remained without any plants as a control plot. A sample of 20g soil was taken from each of 4 pots in the plots every two weeks during summer and nematodes were extracted with Baermann funnels. The density of P. vulnus averaged 998. 9 (±630. 2 SD) just before the experiment. Irrespective of the treatment, the mean density of nematodes rapidly decreased and was highest in the strawberry plot during summer. However, from late August the nematode density decreased slowly in the marigold plot but not in the control and strawberry plots. The density was significantly lower in the marigold plot compared with those in the control and strawberry plots when a sample of 20 g soil was taken from each of 60 pots at the final sampling date two weeks before transplanting strawberries. In the marigold plot nematode numbers decreased to 5.6% of the initial density while in the control and strawberry plots the decrease was to 13.0 and 14.6%, respectively. During summer a decrease in nematode densities was found in all plots, suggesting that climatic factors such as temperature may affect nematode numbers. On the other hand, the difference in nematode density in the marigold plot between late August and early October indicates that marigolds may suppress only 38% of the nematode population. From these results it is concluded that marigolds, African tall, are not effective in controlling P. vulnus in strawberry fields.

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