|Abstract: ||Increased nitrogen applications to crops influence plant - insect interactions and potentially increase insect herbivore population growth. The effect of nitrogen applications on the interactions between a wheat host plant, Triticum aestivum, and its herbivores, Sitobion avenae (Fabricius, 1775) and Rhopalosiphum padi (Linnaeus, 1758) was investigated. Adult body weight, fecundity, maturity rate and longevity of the aphids were measured under different nitrogen applications on four wheat varieties, Solstice, Einstein, Deben and Alchemy. Adult body weight of R. padi on Alchemy and of S. avenae on Deben was lowest. Fecundity of R. padi was least on Deben and Alchemy while fecundity of S. avenae was least on Alchemy. Days to reach maturity were same on all varieties in S. avenae but in R. padi, Deben positively affected the days to reach maturity. Longevity of S. avenae and R. padi was the same on all varieties. N-fertilizer had a positive effect on the adult weight achieved by S. avenae and R. padi. Fecundity and longevity of both species was also positively correlated with N-fertilizer application. In R. padi, nitrogen fertilizer addition reduced the time to reach maturity but not in S. avenae.
Host plant nutritional quality can effectively change parasitoid fitness. The aphid-parasitoid relationship provides an ideal system to investigate tritrophic interactions and assess the bottom up forces operating at different concentrations of nitrogen applications. The effect of varying nitrogen fertilizer applied to the wheat plants used for rearing of S. avenae and R. padi on the performance of Aphidius colemani and Aphidius rhopalosiphi was measured. Percent parasitism and percent emergence were positively correlated with the nitrogen fertilizer treatments. Developmental duration (time taken for egg hatching, larval and pupal stages) was shorter at higher fertilizer levels but the effect was not significant in either species. In males and females of both parasitoid species, adult longevity, however,
increased with the increase of fertilizer. Adult life span of males was significantly affected by the addition of fertilizer, while female lifespan was not affected. Size of both parasitoid species was significantly correlated with the nitrogen fertilizer treatments. Female A. rhopalosiphi were longest at the highest nitrogen fertilizer treatment.
Nitrogen fertilizer significantly affected the number of aphids consumed by H. axyrids. More Rhopalosiphum padi (63.07±1.54) and Sitobion avenae (49.10±0.94) were consumed by adult Harmonia axyridis at the lowest level of fertilizer treatment. Aphids were bigger at higher fertilized plants which ultimately resulted in the fewer aphids being consumed but total bio-mass consumed by H. axyridis on all nitrogen fertilizer treatments were statistically same. Logistic regression analysis of the proportion of prey consumed demonstrated that all developmental stages of H. axyridis exhibited the II type functional response on all nitrogen fertilizer treatments. The rate of successful search (a') of 3rd, 4th instars and adults were (0.042/hour) the same across all fertilizer treatments suggesting that nitrogen fertilization did not affect a'. Maximum handling time for the 1st instars of H. axyridis on R. padi (3.81/h) and S. avenae (4.59/h) was on the highest nitrogen treatment while minimum handling time was for the adults of H. axyridis on R. padi (0.20/h) and S. avenae (0.20/h) on the lowest nitrogen treatment. Handling time varied at varying fertilizer treatments within all instars and affetced the predator's efficiency. The functional response curve, rate of successful search and handling time provides 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 fertilizer regime in the field crops.
The functional responses of all instars of green lacewing, Chrysoperla carnae (Neuroptera: Chrysopidae) to the cereal aphids, R. padi and S. avenae at varying fertilizer
levels was calculated under laboratory conditions. Logistic regression model analysis revealed that all instars showed decelerating curve type II response. The Holling disc equation was used to analyse attack rate (a) and handling time (Th). Different values of Th at varying fertilizer levels within the same instar showed that the predator's efficiency against the prey was affected by host plant nutritional quality. At varying fertilizer treatments, prey densities of 1, 2, 4, 8, 16, 32, 64 and 128 were offered to 1st and 2nd instars and an additional treatment of prey density of 256 for the 3rd instar larvae. Maximum predation of first, second and third instars larvae of C. carnea was 8±1.6, 32±6.4, & 72±5.2 for R. padi and 7±0.8, 31±4.2 & 61±4.4 for S. avenae respectively, on the lowest fertilizer application. Within the same instar fewer aphids were consumed as the level of fertilizer application increased. The number of aphids consumed by the green lacewing, its larval duration, pupal weight, pupal duration, fecundity and male and female longevity were significantly affected by the level of Nitrogen fertilisation to the aphid host plants, except for pupal duration when fed on S. avenae. This study showed that quality of food supplied to the larvae affects the growth and performance of adult green lacewing.
Proportion of both aphid species killed in the experiment was increased by increasing the Mycotal® (Verticillium lecanii) concentration. Nitrogen fertilizer application did not significantly affect the LC50 against the aphids reared at lower and higher nitrogen application. Verticillium lecanii infected aphids (both in S. avenae and R. padi) were less parasitized by Aphidius colemani, when aphids were first treated with V. lecanii and then exposed to A. colemani but the the effect of Mycotal® application was not significant in either specie. However, the emergence of adults from parasitized mummies was, however, significantly lower in infected aphids than uninfected aphids when the aphids were first exposed to the parasitoids and then treated with fungi. Female sex ratio in the emerging adults was significantly decreased in V. lecanii treated aphids in both species. When aphids
were first treated with V. lecanii 72 hours before predation, fewer aphids in both speices were consumed by H. axyridis.|