Low, medium and high densities of the western flower thrips, Frankliniella occidentalis (Pergande), were established in three rooms of a greenhouse to analyze the effect of thrips densities on fruit damage to strawberry plants. Thrips densities were monitored weekly by counting the total number of thrips on flowers, immature fruit and mature fruit, and using blue sticky traps. Fruit damage index was calculated with 4 damage categories for mature fruits. On flowers and fruits (immature and mature), the dominant stages of thrips were adult females and larvae, respectively. Multiple regression analysis of the damage index for larvae, adult females and males on mature and immature fruits 1 week and 2 weeks before harvest showed that fruit damage to mature fruit was significant for larvae on mature fruits (R2=0.775). Damage index (y) transformed to a logistic transformation also showed a significance in terms of percentage (x) of flowers infected with adults (y=0.00958x−0.847, R2=0.627); the control threshold was 21.6% for a damage index of 5. For estimation at this threshold, the sample size was 59.4 flowers.
The species composition of leaf-rolling microlepidoptera on three species of wild cherry, Prunus verecunda, P. apetala, and P. grayana, was studied in a copse in Sendai, Japan. The microlepidopteran communities were monitored once a week from April 2004 to July 2004. A total of eight species belonging to the families Tortricidae and Gelechiidae were collected. On P. verecunda, Eudemis profundana, Archips xylosteanus, A. fuscocupreanus, A. viola, Compsolechia anisogramma and Carbatina picrocarpa were collected, and E. profundana was the dominant species (mean no. of larvae/100 shoots=3.80). On P. apetala, Olethreutes moderata, A. xylosteanus and C. anisogramma were collected. However, the density of each leaf roller on P. apetala was quite low (mean no. of larvae/100 shoots=<0.13). On P. grayana, although only Rhopobota naevana was collected, its density (mean no. of larvae/100 shoots=16.80) was highest among the eight microlepidopteran species collected in this study. These results clearly show that the species composition of leaf-rolling microlepidoptera among the three wild cherry species is quite different, although the three cherries are sympatric species.
In order to clarify the relationship between the number of rice bugs, Leptocorisa chinensis, and number of pecky rice grains, we released eggs of the rice bugs, number ranging from 6 to 36, into 1.6×1.6×1.5 m cages covered with 1 polyethylene mesh (1 mm) at the milk ripening stage in a rice field. The numbers of hatched nymphs were estimated from the remaining egg shells. The actual number of rice bugs was counted at harvest time. Damaged pecky grains having a thickness of over 1.8 mm were counted in each cage after harvest. The number of pecky grains correlated highly with the number of hatched nymphs and rice bugs at harvest time, but not with the number of eggs released. The number of hatched nymphs correlated highly with the number of rice bugs at harvest time, but not with the number of eggs released. The egg survival rate was bimodal with peaks at 0–20 or 80–100%, suggesting that predators caused an all-or-none type mortality to the eggs masses. These results indicate that the estimation of the density of hatched nymphs is more important than those of immigrant adults in order to obtain a highly accurate estimation of pecky rice damage in rice fields.
We developed the mass production procedure of Diaeretiella rapae, a biological control agent against pest aphids. Twenty female adults of D. rapae were introduced into a cage containing Lipaphis erysimi on two plants of komatsuna Brassica rapa nothovar. kept at 20°C and a 15L–9D light cycle. Five treatments were used (i.e., 50-A, 100-A, 50-B, 75-B, 100-B); differing in the number of viviparous female adults provided (50, 75, 100) and interval between introduction of aphids and parasitoids (A: 0 day, B: 3 days). The newly mummified aphids (mummies) were counted and removed on days 7–9, 13–15 and 18–20 after aphid release, and the emergence rate and sex ratio of the parasitoids were assessed by rearing the mummies under the same conditions. From these results, the number of female parasitoids produced was calculated to be largest in 50-B (768), followed by 75-B (719), 100-A (584), 50-A (446) and 100-B (400). A simulation model for producing mummies developed from the life history traits of D. rapae and L. erysimi was used to analyze the present results.
Aphidius gifuensis Ashmead were released on qing-geng-cai plants infested with Myzus persicae (Sulzer) in small experimental greenhouses. Experiments were conducted from September to October 2000 (Trial 1), from November to December 2000 (Trial 2) and from April to May 2001 (Trial 3). Three, six and one adult aphids per plant were introduced on the first days of trials 1, 2 and 3, respectively. Each trial consisted of three treatments: four-time releases of one female and one male adult parasitoid per plant with 7-day-intervals in Greenhouse A, four-time releases of one female and one male per plant with 3-day-intervals in Greenhouse B and a single release of four females and four males per plant in Greenhouse C. In Trial 1, aphid populations were suppressed in all of the greenhouses although they increased by 28.3 per plant in Greenhouse A, 58.0 in Greenhouse B and 117.1 in Greenhouse C on day 17 after the trials started. Trial 2 resulted in an exponential increase of the aphid populations in all of the greenhouses. In Trial 3, the aphid population was controlled only in Greenhouse B. These results show that A. gifuensis can be an effective natural enemy to control the M. persicae population in greenhouses. The effects of release frequency and interval of A. gifuensis, and the initial density of the aphid population on the performance of parasitoids were discussed.