In recent years, Apolygus lucorum has caused increasing damage to cotton and fruit trees in China. The salivary enzymes secreted by A. lucorum when sucking on host plants induce a series of biochemical reactions in plants, and the pre-oral digestion benefits the bug feeding. In this study, the food intake of A. lucorum from 1st instar nymphs to adults was measured, and the corresponding salivary activity of pectinase, amylase, cellulase, protease, polyphenol oxidase and peroxidase was determined. Daily food intake varied with developmental stage, peaking in 3rd and 4th instar nymphs. Pectinase, amylase, cellulase and protease were detected in both nymphal and adult saliva of A. lucorum, while neither polyphenol oxidase nor peroxidase was detected. Protease activity varied with food intake peaking at the 3rd-4th instar, and then slightly decreasing at the 5th instar. Levels of pectinase, amylase and cellulase increased significantly with the daily feeding level until the 3rd instar, corresponding with increasing damage to host plants. The activity of both cellulase and protease had a significant linear relationship with the average daily food intake. The increasing activity of enzymes in saliva explain stage-specific impacts of A. lucorum on the host plants, and suggest that optimal management of A. lucorum would be confined to its control threshold prior to the peak of daily feeding in the 3rd instar.

Sugar beet (Beta vulgaris ssp. vulgaris), a key crop for sugar production, faces significant yield losses by the green peach aphid Myzus persicae (Sulzer) and black bean aphid Aphis fabae (Scop.) as well as the transmission of virus by these aphids. The restriction on the use of neonicotinoids in Europe has intensified this problem, emphasizing the urgent requirement for breeding resistant crop varieties. This study evaluated sugar beet germplasm for resistance against these two aphid species. We investigated the performance and feeding behavior of both species on 26 sugar beet genotypes, using leaf disk bioassays and electropenetrography studies. Additionally, whole plant bioassays and semi-field experiments were carried out with M. persicae. Our findings demonstrate the presence of temporal resistance against both aphid species in the primary sugar beet gene pool. We also found that carrying the beet yellows virus (BYV) enhanced the aphids’ performance. Different plant defense mechanisms were involved: resistance against M. persicae primarily occurred prior to the stylet reaching the phloem, particularly in sugar beet line G3. While resistance against A. fabae turned out to be predominately phloem-located. A high incidence of black stomach symptoms in aphids, exceeding 85% across all genotypes, signified a general incompatibility between sugar beet and M. persicae. In conclusion, sugar beet resistance against aphids involved different mechanisms and was aphid-specific. The identification of these mechanisms and interactions represents a crucial milestone in advancing the breeding of sugar beet varieties with improved resistance.

Insect herbivores can directly affect plant reproduction by feeding on reproductive tissues, or indirectly by feeding on vegetative tissues for which plants are unable to compensate. Additionally, early arriving herbivores may have cascading effects on plant fitness by altering the richness and abundance of the later arriving community. Studies are divided on whether herbivory early in the development of plants can impact plant fitness and whether these effects are predominantly mediated through changes in plant development or subsequent herbivory. Obtaining insight into the generality and consistency of mediated effects on plant reproduction induced by early-season herbivory requires a comparative approach across plant species and environmental conditions. By excluding the herbivore community in an exclosure experiment and by manipulating early-season herbivory in a common garden experiment replicated across four Brassicaceae species and two years, we tested whether early-season herbivory could affect plant development, reproduction, and the herbivore communities associated with individual plants. In addition, we tested a causal hypothesis to disentangle the direct effect of herbivory on plant reproduction, and their indirect effect through changes in the development of plants. Early-season herbivory affected plant development and reproduction, but effects were highly dependent on the plant species, the inducing herbivore species, and the biotic and abiotic environment. The exploratory path analysis indicated that plant reproduction was best predicted by variation in plant development, explaining up to 90.88% of the total effect on seed production. Even though the richness and abundance of the subsequent herbivore community were conditionally affected by the initial colonising herbivore, herbivore pressure is predicted to have only minor effects on reproduction. Importantly, the effects of herbivore pressure on seed set were not direct but were mediated by changes in plant development. Synthesis: Early-season herbivory has the potential to affect plant reproduction through changes in the development of plants and, less strongly, through effects mediated by the plant-associated herbivore community. However, as plants are often able to compensate and attenuate the effects induced by herbivory, the detection, sign, and strength of effects are highly dependent on the plant species and environment.