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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 (F.) and Aphidius rhopalosiphi (D.) reared on Rhopalosiphum padi (L.) were measured. Parasitism and percent emergence of parasitoids 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.

Predator–prey interactions are influenced by nitrogen availability. Wheat (Triticum aestivum cv. Solstice) plants were provided with four levels of nitrogen and examined the responses of coccinellid predator, Harmonia axyridis to cereal aphids, Rhopalosiphum padi and Sitobion avenae. Increased nitrogen application improved nitrogen contents of the plants and also the body weight of cereal aphids feeding on them. In no‐choice feeding trials, H. axyridis consumed more aphids on low fertilised plants, suggesting a compensatory consumption to overcome reduced biomass (lower aphid size). Total biomass devoured by H. axyridis on all nitrogen fertiliser treatments was not statistically different. Logistic regression analysis of the proportion of prey consumed demonstrated that all developmental stages (larval and adult) of H. axyridis exhibited the type II functional response on all nitrogen fertiliser treatments. The rate of successful search (a′) of third and fourth instars and adults were the same across all fertiliser treatments suggesting that nitrogen fertilisation did not affect a′. Maximum handling time for the first instars of H. axyridis on R. padi (3.81 h−1) and S. avenae (4.59 h−1) was on the highest nitrogen treatment while minimum handling time was for the adults of H. axyridis on R. padi (0.20 h−1) and S. avenae (0.20 h−1) on the lowest nitrogen treatment. Handling time varied at varying fertiliser treatments within all instars and affected the predator's efficiency. The functional response curve, rate of successful search and handling time provide the information needed to understand the predator–prey interaction between H. axyridis and these cereals aphids. This could lead to the development of a better strategy for the biological control of R. padi and S. avenae at any particular level of nitrogen fertiliser regime in the field crops.

IntroductionAbiotic stresses drastically affect crops by minimizing yield. Plants face many distinctive abiotic stresses at different stages of plant growth and development. Among these stresses, salinity is prejudicious, limiting plant growth and productivity. Salt stress causes various deleterious effects on morphological, physiological, biochemical, and nutritional attributes. The formation of reactive oxygen species (ROS) is the significant consequence of salt stress. The major ROS comprise hydroxyl ( . OH), superoxide (O 2 .-), and hydrogen peroxide (H 2 O 2 ). The high ratios of ROS are responsible for lipid peroxidation (LPO) of tissues and cause deterioration of proteins, green pigments, and DNA (Schutzendubel and Polle, 2002). However, nature has equipped all vegetations with a defensive antioxidant system to counter the oxidative damage caused by ROS (Apel and Hirt, 2004). Plant growth under saline conditions is highly associated with its antioxidant activities, i.e. superoxide peroxidase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), and glutathione reductase (GTR) activities. High antioxidant activities will inhibit LPO by eliminating the ROS.Silicon (Si) is an excellent growth promoting agent. Silicon is reported to increase plant growth and stimulate productivity in various crop plants. Si application strengthened the plant biomass, height, and productivity under different stressed conditions (Ahmad et al., 2007). It also triggers growth by providing strength and extensibility to plant cells. It strengthens the endoderm of the mature basal cells and improves the extensibility of apical cells of the roots; thus Si contributes to a strong, extensive, and deep root system. However, it is also a vital element that has an excellent tolerance enhancing potential against abiotic Abstract: A pot culture study was conducted to assess the ameliorative effect of silicon, Melia azadirachta leaf extract, and sugar beet root extract, each applied individually or in different combinations, on salinity-induced detrimental effects in pea (Pisum sativum L.). Salinity markedly inhibited the growth, various gas exchange attributes, total phenol contents, membrane stability index, and productivity. On the other hand, lipid peroxidation, electrolyte leakage, H 2 O 2 content, antioxidant activities, and leaf free proline and glycinebetaine contents were significantly enhanced by salinity. However, exogenously applied Si and phyto-extracts markedly alleviated the salinity-induced drastic effects on growth, gas exchange attributes, and productivity. Both phyto-extracts supplemented with silicon gave highly salinity mitigating effects by markedly improving growth, gas exchange attributes, enzymatic activities, osmolytes, and yield. The phyto-extracts and Si suppressed lipid peroxidation, electrolyte leakage, and H 2 O 2 content by strengthening the enzymatic and nonenzymatic (proline and glycinebetaine) antioxidant defense system. The phyto-extracts and Si application also che...

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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.