| Abstract: | The mermithid parasite, Agamermis unka, is the most important natural enemy of the brown planthopper (BPH), Nilaparvata lugens, in Korean rice fields. Very little is known about many aspects of the mermithid's life cycle and behavior, and a study was undertaken to close the data gap. The sex ratio of A. unka isolated from field-collected BPH showed a strong female bias. Even when several A. unka occurred within a BPH, the majority were females. Similarly, the sex ratio of field-collected A. unka adults that were in the soil was strongly biased towards females and, in many instances, the females were found in the absence of males. Females collected from the field from January to May and maintained in water at 25 C had a mean pre-oviposition period of 17-28 days and a mean oviposition period of 17-37 days, and averaged 543-1851 eggs/female. The eggs averaged 20, 17 and 36 days to hatch at 30, 25 and 20 C respectively, but none hatched at 15 C. Most of the eggs (96%) hatched at 25 and 20 C, but only 64% hatched at 30 C. Agamermis pre-parasites could be found on rice stems in the field and laboratory. In the field, BPH-susceptible and BPH-resistant rice cultivars showed no significant difference in the numbers of pre-parasites on the stem. In the laboratory, the number of pre-parasites recovered/rice stem was significantly higher in two out of three trials when BPH nymphs and adults were present. When BPH nymphs were exposed to the pre-parasites in the laboratory, 39% of the brachypterous females and 4.5% of the brachypterous males were parasitized, whereas 0.3% of the macropterous females and 0% of the macropterous males were parasitized. The parasitism data obtained under field conditions showed similar trends. The reason(s) for these differences in parasitism between the BPH sex and wing types that have been observed both in the laboratory and field is unknown. Because brachypterous males and macropterous females and males occur in lesser numbers than the brachypterous females, this may, in part, account for the differences in parasitism observed. However, BPH behavior cannot be discounted as a factor in the differential parasitism. |
