Search

Generalist aphid parasitoids seem to behave as specialists at the local scale. The temporal pattern of host exploitation by parasitoids is key to understand this result. We investigated the temporal dynamics during twenty-one consecutive months of host exploitation by Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae), one of the most generalist aphid parasitoid, in cultivated and uncultivated areas. We applied two complementary approaches: molecular detection of parasitism within living aphids and rearing of aphid mummies collected in Brassicaceae crops and adjacent areas. We built a Maximum Likelihood tree to determine the influence of host plant and aphid species on the genetic structure of D. rapae. We confirmed the existence of a cryptic species developing on Hayhurstia atriplicis (L.) (Hemiptera: Aphididae), but we found no other host-related genetic variation. D. rapae exploited almost exclusively Brevicoryne brassicae (L.) (Hemiptera: Aphididae), a pest of Brassiceae and rarely other hosts. When the crop is absent, D. rapae parasitized B. brassicae on wild Brassicaceae and cover crop instead of alternative aphid host on other plants. These results show the limited role of uncultivated habitats as a reservoir for D. rapae and the interest of wild Brassiceae and cover crops to enhance the control of B. brassicae. [ABSTRACT FROM AUTHOR]

Plant viruses transmitted by vector pests are one of the most important worldwide threats to global food production and security. Biological control strategies to enhance natural enemies (parasitoids and predators) have mainly focused on their ability to reduce pest density. In contrast, few studies have examined how natural enemies affect the spread and the incidence of viruses in a crop, although those results could be used as levers for a more sustainable management of viral diseases. Vector-borne plant viruses can be classified in three categories based on their transmission mode: non-persistently transmitted viruses, semi-persistently transmitted viruses and persistently transmitted viruses, whereas vector density, fitness and movement were identified as main drivers of virus spread in a crop, their relative contributions to virus epidemiology may also depend on both the transmission mode and the presence of natural enemies. The first part of the review focuses on virus transmission dynamics in relation to vector activity and density. Because we identify different patterns for each type of plant viruses, control strategies that lead to changes in vector traits, should be adapted to the targeted virus. However, biological control of insect vectors has been rarely adapted to the mode of transmission of the target virus. Thus, the last part of the review explores the conditions required for natural enemies (parasitoids and predators) to prevent epidemics outbreaks of each type of plant viruses. Briefly, if combined with other practices, biological control of vectors to keep virus incidence below the economic threshold is a promising approach for persistently transmitted viruses but might be more difficult to achieve with non-persistently transmitted viruses and semi-persistently transmitted viruses. [ABSTRACT FROM AUTHOR]

Natural pest control is a significant service supporting agricultural production. However, the relative contribution of several functional groups of natural enemies to natural pest control is unknown for many crop-pest systems. Furthermore, the output of this regulation in terms of damage reduction is rarely assessed. In this study, based on three field experiments, we quantified the effect of ground dwelling predators and parasitoids on two functional groups of broccoli pests. Relying on physical exclusion, we showed that ground dwelling predators significantly lowered the abundance of both the cabbage root fly (Delia radicum) and aphids (Brevicoryne brassicae and Myzus persicae). Ground dwelling predators made a significant contribution to natural pest control, as they lowered pest populations by 37% on average. Parasitoids suppressed 22% of pest populations. Our results suggest that intraguild predation was not an issue since ground dwelling predators did not have a negative impact on the level of natural pest control by parasitoids. Finally, early predation by ground dwelling predators on the cabbage root fly, the most harmful pest in this study, reduced damage in a highly infested context, meaning that natural pest control can effectively support crop production. These results strongly suggest that agricultural practices limiting soil disturbances or even favoring ground dwelling predator overwintering or colonization could lead to fewer damage and losses for farmers. [ABSTRACT FROM AUTHOR]

For many crops, we have poor knowledge about the relationship between pest density and damage. However, investigating pest harmfulness is particularly relevant currently in the search for alternative crop protection strategies that are unlikely to totally suppress pest populations. Here, we assessed the harmfulness of Delia radicum (L.) (Diptera: Anthomyiidae) on broccoli (Brassica oleracea var. italica Plenck). We worked inside insect-proof cages set up in the field with additional pitfall traps to remove ground-dwelling predators. Plants were manually infested with 10 levels of pest density ranging 0–100 individuals per plant, following a natural infestation pattern. Surprisingly, no plants died but almost 100% of the pests introduced died over the course of the experiment. However, all broccoli development and growth traits were negatively correlated with pest density and broccoli head mass at harvest decreased linearly with pest density. The observation over time of development and growth traits showed evidence of plant compensation, suggesting that the head mass of individual plants may have reached similar values if allowed to fully mature. The relationship between pest density and damage, together with forecast models of pest population dynamics could be used to develop decision support tools assessing the relevance of preventative treatments. [ABSTRACT FROM AUTHOR]

The efficiency of the biological control strategies based on parasitoids, either as a release or a conservative measure, depends on the timely arrival or release of the parasitoids after colonization of the field by the pest. In this study, the effects of the delay between the cabbage aphid Brevicoryne brassicae (Hemiptera: Aphididae) infestation and the release of its parasitoid Diaeretiella rapae (Hymenoptera: Braconidae) and of the number of parasitoid releases on aphid population dynamics and host plant growth were investigated. When the parasitoids were released at the start of the infestation, 89.6 % of the aphids were parasitized one month later, and the damage to cabbage was low. The identical number of parasitoids released two weeks after the start of the infestation partially controlled the aphid population, but the growth of the cabbage was altered. The earliest introduction of the parasitoid to the crop led to an efficient host regulation, and the release in three times rather than in one time did not improve the control efficiency at this release delay. The implications for biological control strategies that use parasitoids of aphids are discussed. [ABSTRACT FROM AUTHOR]

Agriculture intensification has deeply modified agroecosystems from field to landscape scales. To achieve successful pest control using natural enemies, understanding species interactions over all scales remains a challenge. Using the cabbage root fly as a model, we studied whether field and landscape characteristics influenced colonization and infestation of broccoli fields by the pest and its control by natural enemies. We also determined whether species of different trophic level or host specialization would respond to environmental characteristics at the same spatial extent. During a multiple-species and multiple-spatial extent study in northwestern France, we recorded pest colonization and infestation in 68 fields, collected associated natural enemies and assessed crop damages. In each field, we considered management practices and characterized the surrounding landscape in 50–500 m-wide buffers. Our main findings are that Delia radicum and its main natural enemies respond to both field and landscape characteristics. Semi-natural areas supported both crop colonization by pests and natural enemy action. The pest and its enemies differed in their responses to field or landscape variables. Landscape elements such as field banks favored the movement of the pest while impeding the movement of some natural enemies. Pest pressure did not increase with the neighboring density of Brassica crops. The presence of natural enemies did not reduce crop damage but reduced pest emerging rates. Finally, specialist parasitoids responded to the landscape at larger spatial extents than generalists. These results outline the complexity of improving pest control through landscape management. [ABSTRACT FROM AUTHOR]

In insect parasitoids, fitness is dependent on the host finding and recognition abilities of the female. Host recognition cues have been described for various host–parasitoid systems, but are still under investigation in aphid parasitoids. Our study aimed to clarify the respective role of physical and chemical cues in recognition of the aphid cuticle. Shed aphid exuviae were used as an elicitor in order to avoid any influence of color, movement, or volatiles present in a living aphid. We assessed the effect of chemical and heat treatments on the texture of the cuticle by using scanning electron microscopy and tested the recognition of treated cuticles by the parasitoid. We showed that recognition cues of the cuticle can be removed chemically (using combined treatments with n-hexane and methanol). Moreover, heat treatment destroyed the physical texture of the cuticle without significantly reducing parasitoid recognition. In a second step, we showed that epicuticular extracts deposited on pieces of paper triggered female attack behavior. First results concerning the chemical composition of the active extract are presented. This study shows that chemical compounds extractable by organic solvents mediate cuticle recognition by aphid parasitoids. [ABSTRACT FROM AUTHOR]

The present study aimed to address how an insect parasitoid makes patch-departure decisions from various types of host patches and how previous patch experiences in the environment modify this decision-making process. Experiments were done with the parasitic waspAphidius rhopalosiphiattacking the grain aphidSitobion avenae. In the experiments, wasps were observed in a laboratory environment containing several patches of various host densities, and behavioural records were analysed using a Cox’s proportional hazards model. Consideration of the effect of the within-patch experience gave a classic pattern of patch-leaving decision rules in parasitoids:A. rhopalosiphiused local information on host quality (i.e. numbers of ovipositions or rejections) and availability (i.e. patch density) to determine departure decision. However, consideration of previous patch experiences provided evidence that these departure rules are fundamentally dynamic, responding to the physiological state of the animal and the information it has about its environment. Results showed thatA. rhopalosiphidecreased its tendency to leave the visited patch after an oviposition. However, when a female has already laid several other eggs in the environment, such an incremental mechanism gradually switched to a decremental one. Hence,A. rhopalosiphiresponded to egg-load depletion by leaving the visited patches sooner and by depositing a smaller number of eggs in those patches, which probably led to a decreased level of superparasitism. Results also indicated that previous experiences enabled wasps to estimate spatial host distribution and then to adjust their behavioural decisions accordingly. Thus,A. rhopalosiphiwas shown to adjust its patch residence time according to the quality and the number of the patches previously visited. These proximate mechanistic rules adopted byA. rhopalosiphifemales are discussed in the context of general predictions from optimality models. [ABSTRACT FROM AUTHOR]