Tritrophic interactions between parasitoids and cereal aphids are mediated by nitrogen fertilizer

Host plant nutritional quality can directly and indirectly affect the third trophic levels. The aphid-parasitoid relationship provides an ideal system to investigate tritrophic interactions （as the parasitoids are completely dependent for their development upon their hosts） and assess the bottom up forces operating at different concentrations of nitrogen applications. The effects of varying nitrogen fertilizer on the performance of Aphidius colemani （V.） reared on Sitobion avenae （E） and Aphidius rhopalosiphi （D.） reared on Rhopalosiphum padi （L.） were measured. Parasitism and percent emergence ofparasitoids were positively affected by nitrogen fertilizer treatments while developmental duration （egg, larval, and pupal stages） was not affected by increasing nitrogen inputs. In males and females of both parasitoid species, adult longevity increased with the increasing nitrogen fertilizer. Hind tibia length and mummy weight of both parasitoid species increased with nitrogen fertilizer concentrations, as a result of larger aphids. This study showed that nitrogen application to the soil can have important consequences for aboveground multitrophic interactions.

Effects of elevated CO2 and plant genotype on interactions among cotton, aphids and parasitoids

Effects of CO2 level （ambient vs. elevated） on the interactions among three cotton （Gossypium hirsutum） genotypes, the cotton aphid （Aphis gossypii Glover）, and its hymenoptera parasitoid （Lysiphlebiajaponica Ashrnead） were quantified. It was hypothesized that aphid-parasitoid interactions in crop systems may be altered by elevated CO2, and that the degree of change is influenced by plant genotype. The cotton genotypes had high （M9101）, medium （HZ401） and low （ZMS13） gossypol contents, and the response to elevated CO2 was genotype-specific. Elevated C02 increased the ratio of total non-structural carbohydrates to nitrogen （TNC ： N） in the high-gossypol genotype and the mediumgossypol genotype. For all three genotypes, elevated CO2 had no effect on concentrations of gossypol and condensed tannins. A. gossypii fitness declined when aphids were reared on the high-gossypol genotype versus the low-gossypol genotype under elevated CO2. Furthermore, elevated CO2 decreased the developmental time of L. japonica associated with the high-gossypol genotype and the low-gossypol genotype, but did not affect parasitism or emergence rates. Our study suggests that the abundance of A. gossypii on cotton will not be directly affected by increases in atmospheric CO2. We speculate that A. gossypii may diminish in pest status in elevated COz and high-gossypol genotype environments because of reduced fitness to the high-gossypol genotype and shorter developmental time of L. japonica.

Genetic variation in foundation species governs the dynamics of trophic interactions

Various studies have demonstrated that the foundation species genetic diversity can have direct effects that extend beyond the individual or population level, affecting the dependent communities. Additionally, these effects may be indirectly extended to higher trophic levels throughout the entire community. Quercus castanea is an oak species with characteristics of foundation species beyond pre- senting a wide geographical distribution and being a dominant element of Mexican temperate forests. In this study, we analyzed the influence of population （He） and individual （HL） genetic diversity of Q. castanea on its canopy endophagous insect community and associated parasitoids. Specifically, we studied the composition, richness （S） and density of leaf-mining moths （Lepidoptera： Tischeridae, Citheraniidae）, gall-forming wasps （Hymenoptera： Cynipidae）, and canopy parasitoids of Q. castanea. We sampled 120 trees belonging to six populations （20/site） through the previously recognized gradi- ent of genetic diversity. In total, 22 endophagous insect species belonging to three orders （Hymenoptera, Lepidoptera, and Diptera） and 20 parasitoid species belonging to 13 families were identified. In general, we observed that the individual genetic diversity of the host plant （HL） has a significant positive effect on the S and density of the canopy endophagous insect communities. In contrast, He has a significant negative effect on the S of endophagous insects. Additionally, indirect effects of HL were observed, affecting the S and density of parasitoid insects. Our results suggest that genetic variation in foundation species can be one of the most important factors governing the dynamics of tritrophic interactions that involve oaks, herbivores, and parasitoids.

Slow-growth high-mortality： A meta-analysis for insects

The slow growth-high mortality hypothesis （SG-HG） predicts that slower growing herbivores suffer greater mortality due to a prolonged window of vulnerability. Given diverse plant-herbivore-natural enemy systems resulting from different feeding ecologies of herbivores and natural enemies, this hypothesis might not always be applicable to all systems. This is evidenced by mixed support from empirical data. In this study, a meta- analysis of the SG-HM hypothesis for insects was conducted, aiming to find conditions that favor or reject SG-HM. The analysis revealed significant within- and between-group heterogeneity for almost a11 explanatory variables and overall did not support SG-HM. In this analysis, SG-HM was supported when any of the following 5 conditions was met： （1） host food consisted of artificial diet; （2） herbivore growth was measured as larval mass; （3） herbivores were generalists; （4） no or multiple species of natural enemies were involved in the study; and （5） parasitoids （i.e., parasitic insects） involved in the study were gregarious. SG-HM was rejected when any of the following 5 conditions was met： （1） herbivores were from the order Hymentoptera; （2） parasitoids from more than 1 order caused herbivore mortality; （2） parasitoids were specialists; （3） parasitoids were solitary; （4） parasitoids were idiobionts or koinobionts; and （5） single species of natural enemy caused mortality of specialist herbivores. All known studies investigated herbivore mortality for a short period of their life cycle. Researchers are encouraged to monitor herbivore mortality during the entire window of susceptibility or life cycle using life tables. Studies involving multiple mortality factors （i.e., both biotic and abiotic） or multiple natural enemy species are also encouraged since herbivores in nature face a multitude of risks during the entire life cycle. More comprehensive studies may increase our understanding of factors influencing the relationships between herbivore growth and mortality.

Effects of the susceptibility of rice varieties to Sogatella furcifera on nymphal development and reproduction of Microvelia horvathi through a food chain

The effects of four rice varieties highly susceptible （ Xiang-Zao-Xian31 and Jia-Yu 164 ）, moderately susceptible （ Wei-You402 ）, and resistant （ Zhu-Liang-You02 ） to the whitebacked planthopper Sogatellafurcifera on the nymphal development, reproduc- tion and survival of Microvelia horoathi were investigated through a food chain using SogateUafurcifera as herbivore prey. Significant longer nymphal developmental time were observed in the 1 st, 4th, 5th instar and whole nymphal stage of M. horvathi raised on S. furcifera fed ZLY02 and WY402 than those of the other two varieties. M. horvathi raised on S. furcifera fed resistant ZLY02 showed a significant lower survival of whole nymphal stage than those fed other three susceptible varieties. Moreover, M. horvathi raised on S. furcifera fed resistant ZLY02 had shorter female longevity and lower lifetime fecundity than those of on susceptible XZX3l. Our results suggest that the S. furcifera- resistant rice variety ZLY02 had a negative effect on the growth and development of the predator M. horvathi through a food chain.

Effect of plant nutrition on aphid size, prey consumption, and life history characteristics of green lacewing

Plant quality can directly and indirectly affect the third trophic level. The predation by all the instars of green lacewing, Chrysoperla carnea （S.） （Neuroptera： Chrysopidae） on the cereal aphids, Rhopalosiphum padi （L.）, and Sitobion avenae （F.） at varying nitrogen fertilizer levels was calculated under laboratory conditions. Wheat plants were grown on four nitrogen fertilizer levels and aphids were fed on these plants and subsequently offered as food to the C. carnea. Aphid densities of 10, 30, and 90 were offered to first, second, and third instar larvae of green lacewing. Increased nitrogen application improved nitrogen contents of the plants and also the body weight of cereal aphids feeding on them. Aphid consumption by green lacewings was reduced with the increase in nitrogen content in the host plants of aphids. Predation of both aphid species by first, second, and third instars larvae of C. carnea was highest on aphids reared on plants with the lowest rate of fertilization, suggesting a compensatory consumption to overcome reduced biomass （lower aphid size）. Total biomass devoured by C. carnea on all nitrogen fertilizer treatments was not statistically different. Additionally, the heavier host prey influenced by the plant nutrition had an effect on the life history characteristics of green lacewings. The larval duration, pupal weight, pupal duration, fecundity, and male and female longevity were significantly affected by the level of nitrogen fertilization to the aphid＇s host plants, except for pupal duration when fed on S. avenae. This study showed that quantity of prey supplied to the larvae affects the prey consumption and thereafter the life history characteristics of green lacewings.

Prey-mediated effects of mCry51Aa2-producing cotton on the predatory nontarget bug Orius majusculus(Reuter)

Genetically engineered(GE)cotton,MON 88702,is protected against cer-tain sucking pests,such as plant bugs and thrips,by producing mCry51Aa2,a mod-ified protein from Bacillus thuringiensis(Bt).Predatory pirate bugs(Orius spp.),natural enemies contributing to biological pest control,are also sensitive to the insecti-cidal protein when exposed continuously to high concentrations.We evaluated effects of MON 88702 on Orius majusculus when fed prey types with different mCry51Aa2 concen-trations.When neonates were provided exclusively Tetranychus urticae spider mites reared on MON 88702(high mCry51Aa2 content),adverse effects on predator survival and de-velopment were confirmed,compared with specimens fed prey from near-isogenic non-Bt cotton.When fed a mixture of T.urticae and Ephestia kuehniella eggs(mCry51Aa2-free),predator life table parameters were similar to the treatment where eggs were fed exclu-sively.When mCry51Aa2-containing spider mites were provided for a limited time at the beginning or the end of juvenile development,effects were less pronounced.While pirate bug nymphs showed similar consumption rates for prey from Bt and non-Bt cotton,choice experiments revealed a preference for E.kuehniella eggs over spider mites.Lepidopteran larvae(Spodoptera littoralis,high mCry51Aa2 content)or cotton aphids(Aphis gossypii,mCry51Aa2-free)reared on MON 88702 as alternative prey did not result in adverse ef-fects on O.majusculus.Our study suggests limited risk of mCry51Aa2-producing cotton for O.majusculus,because its sensitivity for the Bt protein is relatively low and its natural food consists of diverse prey species with varying concentrations of Bt protein.

Internal dispersal of seed-inhabiting insects by vertebrate frugivores:a review and prospects

The finding that some seed-inhabiting insects can survive passage through the entire digestive tract of seed-dispersing vertebrates is relatively recent,but evidence suggests that it does occur.Here,I document this phenomenon,discuss its qualitative and quantitative dimensions,and offer suggestions for further research.The few documented cases that I review include plant species belonging to different families,with varied fleshy fruit types,number of seeds per fruit and seed size.The vertebrate frugivores involved include passerines that feed on relatively small fruits,and galliforms,and perissodactyls and primates that feed on larger fruits.The seed-inhabiting insects in-volved are the larvae of seed-infesting wasps,parasitoid wasps and seed-infesting beetles.The phenomenon has been verified in open,rural ecosystems in North America and Southern Europe,and in tropical and subtropical forests in South America.These varied scenarios suggest that the qualitative dimension of the phenomenon is considerably greater than known thus far.A simple method for detecting new events is proposed.However,re-search must also focus on the identity and biology of seed-feeding insects of wild fleshy fruits and their parasitoids.High survival rates of seed-inhabiting insects after vertebrate gut passage are predominant.This phenomenon generally appears to favor insect dispersal.

Effects of plant trichomes on herbivores and predators on soybeans

Tritrophic interaction in soybean system has received increasing attention recently. However, few studies have investigated the influence of plant trichomes on the population dynamics of soybean herbivores and their natural enemies. We conducted a field survey to investigate whether soybean trichomes affected the abundance of herbivores and their predators. The results of this study show that moderately or densely pubescent trichomes have positive influences on the abundance of some herbivores （e.g., Stollia guttiger） and predators （e.g., Propylaeajaponica and Orius similes） although the influence may change over time, while trichome types do not affect the density of soybean aphid, Aphis glycines.