Salicylic acid (SA), a phytohormone, has been considered to be a key regulator mediating plant defence against pathogens. It is still vague how SA activates plant defence against herbivores such as chewing and sucking pests. Here, we used an aphid-susceptible wheat variety to investigate Sitobion avenae response to SA-induced wheat plants, and the effects of exogenous SA on some defence enzymes and phenolics in the plant immune system. In SA-treated wheat seedlings, intrinsic rate of natural increase (rm), fecundity and apterous rate of S. avenae were 0.25, 31.4 nymphs/female and 64.4%, respectively, and significantly lower than that in the controls (P < 0.05). Moreover, the increased activities of phenylalanine-ammonia-lyase, polyphenol oxidase (PPO) and peroxidase in the SA-induced seedlings obviously depended on the sampling time, whereas activities of catalase and 4-coumarate:CoA ligase were suppressed significantly at 24, 48 and 72 h in comparison with the control. Dynamic levels of p-coumaric acid at 96 h, caffeic acid at 24 and 72 h and chlorogenic acid at 24, 48 and 96 h in wheat plants were significantly upregulated by exogenous SA application. Nevertheless, only caffeic acid content was positively correlated with PPO activity in SA-treated wheat seedlings (P = 0.031). These findings indicate that exogenous SA significantly enhanced the defence of aphid-susceptible wheat variety against aphids by regulating the plant immune system, and may prove a potential application of SA in aphid control.

Asexual reproduction is very common in invasive insect pest. In the recent years, increasing evidences have shown that some invasive asexual lineages display an outstanding capacity to predominate in space and persist on time (superclones). However, little is known about the host-use behavior of these superclones. The English grain aphid Sitobion avenae (Fabricius) (Hemiptera: Aphididae) is one of the major pests of cereals worldwide. Chilean populations of the grain aphid are characterized by a high degree of heterozygosity and low genotypic variability across regions and years, with only four predominant superclone genotypes representing nearly 90% of populations. In this study, (1) the reproductive performance and (2) the probing behavior followed a host shift of one superclone and one non-superclone of S. avenae, were compared. The host plant in the superclone did not affect the reproductive performance, while in the non-superclone was lower on highly defended wheat seedling. The experimental switching of the host plants from barley (without chemical defenses) to two wheat species with low and high levels of chemical defenses, revealed that superclone exhibited a flexible probing activities related to access of sieve elements, while the non-superclone exhibited rigid responses. These findings are consistent with the pattern of occurrence of these genotypes in the field on cereals with different plant defenses (e.g. benzoxazinoids). These responses are discussed on the view of developing new strategies for the management in invasive populations of aphid pest species.

Clones have been in the forefront of biological interest for many years. Even so, open discussions continue to surround the concept of clonality, which has been recently much debated in the scientific literature, both in terms of philosophical meaning as well as empirical determination. Philosophically, the clone is the horizontally produced lineage from a single fertlized egg (e.g. mammals by division of the fertilized egg and representing a single generation) or vertically produced offspring (e.g. aphids representing different successive generations) from a single asexual stem mother (originally for a particular lineage, following hatching of the overwintering sexual egg in the spring); empirically, the aspect of genetic fidelity is also considered important, so-called clones being assumed to have an identical genome among clone mates. In reality of course, such members of a clonal lineage must differ at various regions of the genome, since mutation is a fundamental property of the DNA itself. Yet few studies have so far set out to show this empirically in eukaryotic organisms, which indulge in periods of asexual reproduction, sometimes, as in aphids, over many generations. In the present study, we have investigated asexual lineages of the grain aphid, Sitobion avenae (F.), a global pest of cereals, over five successive generations employing AFLP-PCR molecular techniques. Our main interest was to see how much variation was present in the early generations and if this variation was transmitted through the asexual lineages. By monitoring AFLP-based polymorphisms, we show that, in this aphid species, of a total of 110 individuals from two lineages tested (termed SA and SB), random mutations (band deletions, more rarely additions) were apparent from the third generation onwards, and although some mutations were found to be transmitted transgenerationally, others were rarely transmitted through the particular lineages they were detected in. Using Arlequin v. 2.0, average gene diversity within the lineages was found to be 0.024 ± 0.013 and 0.031 ± 0.016 for SA and SB, respectively. It was also found from the rearing of the lineages that one lineage, SA, was more fecund than the other lineage, SB, over the five generations (N = 818 vs. N = 358 total stem mothers plus nymphs for the two lineages, respectively).

Both Sitobion avenae (Fabricius) and Rhopalosiphum padi (Linnaeus) are the most important pests of wheat in China and usually coexist on the late period of wheat growth. Pirimicarb was introduced into China for wheat aphid control in early 1990s, and differential susceptibilities of Sitobion avenae (Fabricius) and Rhopalosiphum padi (Linnaeus) to pirimicarb have been observed. A bioassay exhibited that Rhopalosiphum padi possessed significantly higher susceptibility to pirimicarb than Sitobion avenae. The addition of synergists DEF, an esterase inhibitor, PBO, a cytochrome P450 monooxygenase inhibitor, and DEM, a glutathione S-transferase inhibitor, resulted in apparent reductions in the differential susceptibilities, suggesting the involvement of the above three detoxification enzymes in the differential susceptibility to pirimicarb between Sitobion avenae and Rhopalosiphum padi. A biochemical analysis showed that the activities of carboxylesterases and glutathione S-transferases were significantly higher in Sitobion avenae than in Rhopalosiphum padi, consistent with the results of synergism. Acetylcholinesterase is the target enzyme of pirimicarb and the sensitivity of acetylcholinesterase to pirimicarb was significantly higher in Rhopalosiphum padi than in Sitobion avenae. The combined results suggest that multiple mechanisms are likely to be responsible for differential susceptibilities to pirimicarb between Sitobion avenae and Rhopalosiphum padi. The results obtained from this study should be helpful in the rational applications of insecticides.

Generalist predators contribute to pest suppression in agroecosystems. Spider communities, which form a substantial fraction of the generalist predator fauna in arable land, are characterized by two functional groups: web-building and cursorial (non-web-building) species. We investigated the relative impact of these two functional groups on a common pest (Sitobion avenae, Aphididae) in wheat by combining a molecular technique that revealed species-specific aphid consumption rates with a factorial field experiment that analyzed the impact, separately and together, of equal densities of these two spider functional groups on aphid population growth. Only cursorial spiders retarded aphid population growth in our cage experiment, but this effect was limited to the initial aphid-population growth period and low-to-intermediate aphid densities. The molecular analysis, which used aphid-specific primers to detect aphid DNA in predator species, detected the highest proportion of aphid-consuming individuals in two cursorial spiders: the foliage-dwelling Xysticus cristatus (Thomisidae) and the ground-active Pardosa palustris (Lycosidae). The results suggest that manipulating the community composition in favour of pest-consuming functional groups may be more important for improving biological control than fostering predator biodiversity per se. Agricultural management practices that specifically foster effective species or functional groups (e.g. mulching for cursorial spiders) should receive more attention in low-pesticide farming systems.

Cereal stands in central Europe are commonly infested with three species of aphids that may become serious pests. With increasing abundance, the proportion of a particular species in the total aphid population may remain constant, suggesting a density-independent exponential growth, or the proportion can change, suggesting density-dependent constraints on growth. The constraints that affect particular species, and thus their relative abundance, were studied. The proportionality between maximum abundances of the cereal aphids was studied using a 10-year census of the numbers of aphids infesting 268 winter wheat plots. For two species their abundance on leaves and ears was compared. With increasing aphid density the maximum abundance of Rhopalosiphum padi (Linnaeus) remained proportional, but not that of Sitobion avenae (Fabricius), which was constrained by the smaller surface area of ears compared to leaves. There was no evidence of inter-specific competition. Maximum abundance of R. padi and Metopolophium dirhodum (Walker) on leaves did not change proportionally as the proportion of M. dirhodum decreased with increasing overall aphid density. This decrease was probably caused by the restricted distribution of M. dirhodum, which is confined to leaves, where space is limiting. No change in proportion between populations was detected when the average densities were below 0.54 aphids per leaf or ear. Non-proportional relationships between aphid populations appeared to be due to spatial constraints, acting upon the more abundant population. Detecting the limitation of population growth can help with the assessment of when density-independent exponential growth is limited by density-dependent factors. This information may help in the development of models of cereal aphid population dynamics.

The relative contributions of host plant, herbivore species and clone to variation in the interaction between cereal aphids and wheat were investigated using five clones each of three species, Rhopalosiphum padi (Linnaeus), Sitobion avenae (Fabricius) and Schizaphis graminum (Rondani), on seedlings of two cultivars of Triticum aestivum L. and one cultivar of Triticum durum Desf. More individuals and biomass of R. padi than of the other two species were produced on seedlings. The three wheat cultivars lost similar amounts of biomass as a result of infestation by aphids, with the amount lost depending on aphid species: S. avenae caused the lowest loss in biomass. Variation in aphid biomass production was due mostly to differences among aphid species (70%), less to the interaction between wheat type and aphid species (7%), and least to aphid clone (1%). The specific impact of the aphids on the plants ranged from 1.7 to 3.7 mg of plant biomass lost per mg of aphid biomass gained, being lowest for R. padi and highest for S. graminum. Variation in plant biomass lost to herbivory was due mostly to unknown sources of error (95%), probably phenotypic differences among individual seedlings, with 3% due to aphid species and none attributable to aphid clone. For these aphid–plant interactions, differences among aphid clones within species contributed little to variation in aphid and plant productivity; therefore, a small sample of clones was adequate for quantifying the interactions. Furthermore, one clone of each species maintained in the laboratory for about 200 parthenogenetic generations was indistinguishable from clones collected recently from the field.

In this study, we evaluated in field trials the effects on non-target species, of transgenic corn producing the Cry1Ab toxin of Bacillus thuringiensis (Bt). In 1998, we collected Ostrinia nubilalis (Hübner) larvae from transgenic Bt corn (Novartis Hybrid 176) and non-Bt corn at four geographical sites. We found a significant variation in parasitism by the tachinids Lydella thompsoni (Herting) and Pseudoperichaeta nigrolineata (Walker) among sites, and more parasitism in non-Bt than in Bt fields. The Bt effect did not vary significantly among fields. In 1999, we performed a field experiment at two sites, comparing the temporal abundance of non-target arthropods in Bt corn (Monsanto Hybrid MON810) and non-Bt corn. The non-target insects studied included the aphids Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.) and Sitobion avenae (F.), the bug Orius insidiosus (Say), the syrphid Syrphus corollae (Meigen), the ladybird Coccinella septempunctata (L.), the lacewing Chrysoperla carnea (Stephens), thrips and hymenopteran parasitoids. For all species but one, the number of individuals varied greatly over the season but did not differ between the types of corn. The only exception was thrips which, at one site, was significantly more abundant in Bt corn than in non-Bt corn. However this difference did not remain significant when we took the multiple tests into account. Implications for pest resistance management, population dynamics and risk assessment are discussed.

The field experiments were conducted during the summer of 1985 to determine the degree of winter wheat resistance to the grain aphid Sitobion avenae. Asta, Grana, and Saga cultivars were generally more resistant than Dana, Liwilla, and Emika. The resistance mechanism involved both antixenosis and antibiosis. Grana was least preferred during aphid migration. Asta, Saga, and, to a lesser extent, Grana had an antibiotic effect on growth and development of the apterous aphids (longer prereproductive period, lower fecundity and lower values of the intrinsic rate of increase, rm, during the first days of reproduction). Moreover, var. Asta showed a “secondary antixenosis” during colonization of ears due to the presence of awns.