Economic thresholds of Thrips palmi (Thysanoptera: Thripidae) for eggplants in a greenhouse
Thrips palmi Karny has recently been implicated in damage to eggplant fruits. This thrips causes cosmetic scars on and deforms fruits, thus lowering their market value. To investigate the relationship between the density of thrips and the resulting damage, experimental plots with initial release densities of 0, 20, 40, 70, and 100 adults thrips per plot were established under greenhouse conditions. Thrips density (for flower sampling = x1; for sticky trap = x2) was monitored using flower sampling and blue sticky trap counts, and was found to be directly related to the proportion of damaged-fruits (y) and the reduction in fruit yield: significant relationships were found for the flower samples (y = 1.2261x1 − 0.6232, r2 = 0.8582) and for the trap catches (y = 11.667 ln(x2) − 9.5, r2 = 0.8896). The proportion of damaged fruits that could be tolerated from an economic perspective, based on the cost of controlling the thrips population chemically and the market value of the fruit, was 6.67–11.76 %; this translated into economic thresholds of 1.05–1.5 adults and/or larvae of T. palmi per flower or 4.91–10.17 adults T. palmi per four-day sticky card count.
Temperature-dependent development and life table parameters of Thrips palmi (Thysanoptera: Thripidae) on eggplant
The life history of Thrips palmi Karny on eggplant (Solanum melongena L.) leaves was studied based on the age stage and two sex-life tables at 16, 19, 22, 25, and 31 °C. The intrinsic rate of increase (r) at these temperatures was 0.0427, 0.0566, 0.0979, 0.1738, and 0.2237 day−1, respectively. The relationship among the gross reproductive rate (GRR), the net reproductive rate (R0), and the pre-adult survivorship (la) is consistent with R0 < la × GRR < GRR for all results at different temperatures. The mean generation time was 47.52, 38.33, 29.52, 19.81, and 13.88 days, respectively. The developments of pre-adult and adult stages were faster in males than in females. The means of developmental periods for each developmental stage decreased with increases of temperature. The maximum life span of female adults was noted at 56.67 days, whereas that of males was 50.66 days at 16 °C. The maximum female fecundity (64.18 eggs/female) was recorded at 25 °C and the lowest (23.38 eggs/female) at 16 °C. Life table data could be used to project population growth, to design mass rearing programs, and to establish management tactics to control insect pests.
Direct Feeding Damage on Cucumber by Mixed-Species Infestations of Thrips palmi and Frankliniella occidentalis (Thysanoptera: Thripidae)
Distributions of Thrips palmi Karny and Frankliniella occidentalis (Pergande) within plants and relative contributions of each species to fruit scarring were investigated in field plantings of cucumber, Cucumis sativus (L.). Densities of T. palmi (number per unit area plant substrate) were greatest on foliage, whereas F. occidentalis densities were greatest on flowers. Densities of both species were lowest on fruits. Both species had secondary sex ratios that were biased towards females. The proportion of male F. occidentalis increased substantially in flower samples. Temporal variation in the incidence of fruit scarring, within and between field plantings, was related to variation in densities of F. occidentalis (but not T. palmi ). Within-field spatial variation in fruit scarring on a given harvest was also associated primarily with variation in F. occidentalis densities. Because small, developing fruits physically support the female flowers, the high densities of F. occidentalis in flowers may create opportunities for them to incidentally feed upon and scar young fruit.