Studies on the mechanism of Pseudaulacaspis pentagona (Targioni) (Hemiptera: Diaspididae) resistance of tea plant and its practical use

The white peach scale, Pseudaulacaspis pentagona Targioni (Hemiptera: Diaspididae), is one of the most serious pest of tea plant Camellia sinensis (L.) O. Kuntze. in Japan. Heavy infestation of P. pentagona causes withering of tea shoots and reduces the yield of tea leaves for some years. Chemical-based IPM has been widely practiced by growers with unsatisfactory results. The use of cultivars that are resistant to the white peach scale has been considered as an alternative control measure, but extensive studies to select promising cultivars have not been conducted. This study was conducted at the Tea Branch of the Miyazaki Agricultural Experiment Station in order to make the use of resistant cultivars practical and to clarify the mechanisms of resistance to insects in tea cultivars. 1. The degree of infestation by P. pentagona on 106 tea cultivars and lines were investigated for 3 years. The degree of infestation varied greatly among the cultivars and there was significant correlation between years in the order of infestation degree, suggesting that the resistance is an inherent trait. The degree of infestation on cultivars derived from hybrids between of the Assam variety, C. sinensis var. assamica (Mast.) Kitam., and the Chinese variety. C. sinensis var. sinensis (L.) Kuntze, was lower than that on cultivars derived from the Chinese variety alone. In addition, resistant cultivars were found at significantly higher ratio in the hybrid cultivars than in the cultivars of the Chinese variety. 2. To evaluate insect resistance of tea cultivars, the life history traits of P. pentagona were investigated in the laboratory by rearing the scale insect on nursery stocks of resistant and susceptible cultivars. In males, the duration of the second larval stadium, the pupal stage, and the period from hatching to adult emergence was significantly longer when reared on resistant than on susceptible cultivars. In females, the second larval stadium, preoviposition period, and the period from hatching to the beginning of oviposition were also significantly retarded when reared on resistant cultivars. Only 20-25 % of neonate female larvae developed into the adult stage on the resistant cultivars. "Hatsumomiji" and "Sayamakaori". whereas 40-60 % of the larvae developed into adults on the susceptible cultivars, "Yutakamidori" and "Takachiho". The estimated fecundity of P. pentagona were expressed as the sum of eggs laid and eggs remained in the ovary, of which the latter was restricted to those reached the final or semifinal stage of egg development at one or two days after beginning of oviposition. The estimated fecundity of females reared on the resistant cultivars, "Sayamakaori" and "Hatsumomiji" was 5 to 10 % of that on the susceptible cultivar, 'Takachiho". The estimated fecundity also approximately coincided with the total number of eggs actually realized during the life time of females for all four populations of P. pentagona reared on different cultivars and under different conditions. These results suggest that antibiosis strongly affect the life history traits of P. pentagona and determine the degree of infestation. 3. The total time of waveform appearance caused by P. pentagona sucking activity was compared between females reared on the resistant (Hatsumomiji) and the susceptible (Takachiho) cultivars using a computer-based electronic monitoring system (EMS), together with observations on the amount of dorsal wax secretion. Nine sorts of distinctive waveform (Ato I-) were observed during the sucking activity. and the total time of each waveform appearance was recorded during 4 hours. The total time of appearance for B-. C-, D-, and E-waveforrns was longer than that for other waveforms, suggesting an association with major sucking activities. In the time of appearance. B-waveform was longer on the susceptible cultivar, C- and E-waveforms were longer on the resistant one, and there was no difference between the cultivars in D-waveform. Wax secretion was observed in almost all females on the susceptible cultivar, while not on the resistant cultivar. 4. The scale cover weight of P. pentagona reared on the resistant cultivars, "Sayamakaori" and "Hatsumomiji" was significantly lighter than the weight of that on the susceptible ones, "Takachiho", suggesting that nutritional ingestion was insufficient in the latter. Sap exudation through the severed stylets of P. pentagona females showed pH of 7.8 or higher, suggesting that it was phloem sap. The proportion of females exuding phloem sap through the severed stylets was significantly lower on resistant cultivars than on susceptible ones. The amount of phloem sap exudation 24 hrs after stylectomy was smaller on the resistant cultivars, "Sayamakaori" and "Hatsumomiji" than on the susceptible one, "Takachiho", This difference is presumably related to the mechanism of antibiosis in tea plant, since the diameters of maxillary and mandibular stylets did not differ significantly among individuals of the scale insect. The amount of phloem sap exudation and the degree of infestation by P. pentagona on 11 tea cultivars showed a significant positive correlation.