Neoseiulus californicus (McGregor) is a potential biological control agent of spider mites in orchards. The field occurrence of this species was first reported from citrus orchards in Jeju Island and recently this mite started to be sold as a commercial product in Korea. However, the natural occurrence of N. californicus in the Korean mainland is unclear or quite limited. At this time, it is not certain whether N. californicus can overwinter in the Korean mainland or not. This mite is presently planning to be used as a biological control agent against spider mites in apple orchards of the Korean mainland. The main objective of this study was to investigate natural overwintering sites of N. californicus and possible factor increasing winter survival of this species, to develop effective method for collecting overwintering population of N. californicus. For the first objective, we conducted field survey in two citrus orchards of Jeju in early and late February 2008. Samples were collected from various possible overwintering sites: citrus trees, ground covers and windbreaks. Total 259 phytoseiid mites were collected, 94% of the mites were identified as N. californicus and 98% of them were adult females. Most of N. californicus were collected from ground covers, specially the weed species which had rosette leaves: Youngia japonica (L.) DC. and Erigeron spp. This result seemed to suggest that N. californicus try to hide beneath weed leaves near ground surface during winter to get successful survival. Thus, the second experiments were conducted in Andong from December 2007 to early March 2008. N. californicus conditioned in a greenhouse with natural day length was individually put in a small tube, and the tubes were kept under different ground surface conditions: bare, fallen leaves and urethane foam. When water had been added in the tube, the mites on bare ground had died after one month, while 70% of the mites of the other cover treatments were surviving. In these cover treatments, 7-10% of the mites had survived until two months, and eventually all died within three months. This result showed that hiding under cover structure near ground surface may be effective to increase winter survival of N. californicus. Therefore, collecting weeds which have rosette leaves may be an effective method to estimate of the overwintering possibility of N. californicus in the Korean mainland.

• Mitsuhiro Kawashima(Insect Ecology Laboratory, School of Bioresource Sciences, Andong National University)
• Chuleui Jung(Insect Ecology Laboratory, School of Bioresource Sciences, Andong National University)