POPULATIONECOLOGY Varietal Screening Based on Demographic Parameters: Resistance of Tea to Brevipalpus phoenicis (Acari: Tenuipalpidae) JOHN S. KENNEDY ANDTHIERRY HANCE Unite d'Ecologie et de Biogeographie, Place Croix du Sud 4-5, Universite Catholique de Louvain, B-1348 Louvain-Ia-Neuve, Belgium Environ. Entomol. 24(6): 1481-1486 (1995) ABSTRACT Host plant resistance to herbivores is usually measured by visual screening in the field, which is very expensive. We tested an alternative method using 26 varieties of tea, Camellia sinensis, selected from different geographical locations to screen for resistance against the red and black Rat mite, Brevipalpus phoenicis (Geijskes); survival and logistically fixed net fecundity schedules (predicted number F I offspring) were compared. Survival schedules of the mites varied among the different varieties, as indicated by a change in the concavity of the survivorship curves and measured in terms of entropy. Varieties were classified according to their level of resistance/susceptibility based on the number of predicted F 1 offspring. The level of susceptibility was positively correlated with life expectancy and was negatively related to entropy. Varieties that evolved from northeast India were consistently less susceptible, which suggests that a search in this region to identifYgenes for resistance might be useful for future breeding programs. KEY WORDS Brevipalpus phoenicis, tea, plant resistance, varietal screening, life table, de- mography RESISTANCEIS USUALLYmeasured by visual screening of plants, which requires large fields plus inputs like fertilizers, pesticides, or man power to establish a good crop to use for screening. Nev- ertheless, many times it is almost impossible to have a crop with an exclusive infestation of the test organism. In addition, perennial crops take a long time to establish. Hence it is worthwhile to com- pare various demographic parameters estimated in the laboratory to measure the degree of host plant susceptibility/resistance. In this context, Lawton and McNeill (1979) showed theoretically that even a small difference in the value of the intrinsic rate of natural increase (rill) could make a big differ- ence in the abundance of herbivore populations. However, comparing values of r", using population projection matrices is very difficult because it is mathematically complex, and differences among host plants are often so small that they are not significant. Moreover, it is valid only after the pop- ulation has attained a stable age structure, a con- dition that is seldom encountered. In this study we used the age-specific cumula- tive net fecundity schedule (.Ilxm x ) (or number of offspring in the F 1 generation) to measure host plant resistance. This value represents the growth of a population, initiated with newly colonizing in- dividuals on uninfested plants. The growth of the population in the beginning favors younger indi- viduals, and the growth rate fluctuates for 150 d before reaching a stable age distribution (J.S.K., unpublished data). The growth rate in the begin- ning is much higher than the classical exponential growth rate, taking the stable age distribution into account. The method developed by Van Impe and Hance (1993) using offspring production in the Fl generation is more realistic and allows rapid screening of different varieties by simply compar- ing the logistically fixed cumulative net fecundity curves of the candidate herbivore on different va- rieties of the host plant. Our main goal was to apply this method for eval- uating the level of susceptibility of different vari- eties of tea, Camellia sinensis (L.) O. Kuntze, orig- inating from different geographical locations, to the red and black flat mite, Brevipalpus phoenicis (Geijskes). Further, we have extended this method by comparing survivorship on different varieties using mite life expectancy and entropy values. Materials and Methods Mites. The population of B. phoenicis used in this experiment was collected from tea gardens of The Nilgris Hills, Tamil Nadu, South India, and mass reared on tea plants (variety: Burundi local), maintained at 26 ± 2°C With 65-75% RH and a photoperiod of 16:8 (L:D) h in a greenhouse. The 0046-225X195/1481-I486$02.00/0 © 1995 Entomological Society of America Downloaded from https://academic.oup.com/ee/article/24/6/1481/499848 by guest on 17 April 2022