Intraguild interactions between the entomopathogenic fungus Pandora neoaphidis and an aphid predator and parasitoid at the population scale

Unhealthy herds and the predator–spreader: Understanding when predation increases disease incidence and prevalence

Disease ecologists now recognize the limitation behind examining host–parasite interactions in isolation: community members—especially predators—dramatically affect host–parasite dynamics. Although the initial paradigm was that predation should reduce disease in prey populations (“healthy herds hypothesis”), researchers have realized that predators sometimes increase disease in their prey. These “predator–spreaders” are now recognized as critical to disease dynamics, but empirical research on the topic remains fragmented. In a narrow sense, a “predator–spreader” would be defined as a predator that mechanically spreads parasites via feeding. However, predators affect their prey and, subsequently, disease transmission in many other ways such as altering prey population structure, behavior, and physiology. We review the existing evidence for these mechanisms and provide heuristics that incorporate features of the host, predator, parasite, and environment to understand whether or not a predator is likely to be a predator–spreader. We also provide guidance for targeted study of each mechanism and quantifying the effects of predators on parasitism in a way that yields more general insights into the factors that promote predator spreading. We aim to offer a better understanding of this important and underappreciated interaction and a path toward being able to predict how changes in predation will influence parasite dynamics.

Do predators keep prey healthy or make them sicker? A meta-analysis

Ecological theory suggests that predators can either keep prey populations healthy by reducing parasite burdens or alternatively, increase parasitism in prey. To quantify the overall magnitude and direction of the effect of predation on parasitism in prey observed in practice, we conducted a meta-analysis of 47 empirical studies. We also examined how study attributes, including parasite type and life cycle, habitat type, study design, and whether predators were able to directly consume prey contributed to variation in the predator-prey-parasite interaction. We found that the overall effect of predation on parasitism differed between parasites and parasitoids and that whether consumptive effects were present, and whether a predator was a non-host spreader of parasites, were the most important traits predicting the parasite response. Our results suggest that the mechanistic basis of predator-prey interactions strongly influences the effects of predators on parasites and that these effects, although context dependent, are predictable.

Direct and Indirect Effect via Endophytism of Entomopathogenic Fungi on the Fitness of Myzus persicae and Its Ability to Spread PLRV on Tobacco

Simple Summary

Aphids are major crop pests that are feeding on plant sap and transmitting plant viruses, thus inducing high yield losses worldwide. As chemical pesticides are decreasingly used in plant protection, fungi that cause disease to insects (entomopathogenic fungi) are one of the promising alternatives. They are commonly applied by spraying plants to protect them against herbivores. When applied, some fungi penetrate and live within plant tissues, thus helping to internally protect from insect attacks and other plant diseases. The aim of our study was to assess the effects of entomopathogenic fungi (EPF) applied firstly by contact after insect direct spraying, secondly by endophytic plant inoculation, and thirdly by associated both methods assessing the green peach aphid performances. The impact of the presence of endophytic entomopathogenic fungi (EEPF) in plant tissues on virus transmission by aphids was also considered. We found that the EPF Beauveria bassiana killed the green peach aphid and reduced its fecundity regardless of the application method. On fungal-inoculated plants, there was also a high mortality of aphid nymphs and infection by the potato leafroll virus (PLRV) was delayed by about a week with the EEPF treatment compared to fungal-free plants. This study showed that spraying plant leaves with EPF not only has a direct insecticidal effect against insects but could also have beneficial side effects for the plant against viruses.

Abstract

Aphids are major crop pests that transmit more than half of all insect-vectored plant viruses responsible for high yield losses worldwide. Entomopathogenic fungi (EPF) are biological control agents mainly used by foliar application to control herbivores, including sap-sucking pests such as aphids. Their ability to colonize plant tissues and to interact with diverse plant pathogenic microorganisms have been reported. In our study, we evaluated the effectiveness of Beauveria bassiana ((Balsamo-Crivelli) Vuillemin) directly applied by contact or/and indirectly via endophytism in tobacco plants (Nicotiana tabacum L.) against the virus vector Myzus persicae (Sulzer) carrying the Potato leafroll virus (PLRV) or not. We found that both contact treatment and endophytic colonization of leaves significantly increased aphid mortality and decreased the fecundity rate when compared to control plants. In addition, on fungal-colonized leaves, viruliferous aphids were more negatively impacted than virus-free ones and nymph mortality was significantly higher than on fungal-free plants. Furthermore, we assessed PLRV transmission by M. persicae on tobacco plants inoculated with either B. bassiana or Metarhizium acridum ((Driver and Milner) JF Bischoff, Rehner, and Humber) as source or/and recipient plants. Myzus persicae was found to acquire and transmit PLRV regardless of the treatment. Nevertheless, the infection rate of endophytically colonized plants was lower at a seven-day incubation period and had increased to almost 100% after fifteen days. These results suggest that B. bassiana is effective against aphids, both by contact and via endophytism, and both B. bassiana and M. acridum delayed PLRV infection in tobacco.

The effect of synthetic female sex pheromone on the transmission of the fungus Metarhizium brunneum by male Agriotes obscurus click beetles

Autodissemination techniques can potentially be used to distribute insecticides, including microbial insecticides, to cryptic pests. This approach is reliant on the target insect either passing the pathogen passively to other insects or the pathogen cycling within the population after the initial host dies. Here we examine, in small scale experiments, whether male Agriotes obscurus click beetles passively transmit the spores of the fungus Metarhizium brunneum directly, or indirectly via the environment, and whether this is influenced by exposure to synthetic female pheromone. We found that the beetles did not avoid M. brunneum spores and that this behaviour was not affected by pheromone. Exposure to pheromone increased beetle movement and uptake of spores, but this did not result in an increase in infected beetles under our conditions. Beetles were able to transfer spores at high levels via environmental contamination. However, contamination of the environment declined rapidly after exposure to the spores. The results are discussed in the context of developing an autodissemination strategy for click beetles.

Interaction Between Beauveria bassiana (Hypocreales: Cordycipitaceae) and Coptera haywardi (Hymenoptera: Diapriidae) for the Management of Anastrepha obliqua (Diptera: Tephritidae)

The interaction between the entomopathogenic fungus Beauveria bassiana (Balsamo) and the parasitoid Coptera haywardi (Oglobin), as potential biological control agents for Anastrepha obliqua (Macquart) fruit flies, was evaluated under laboratory and semi-protected field cage conditions. The effects of the parasitoids and fungus were individually and jointly assessed in Plexiglas cages. Application of B. bassiana dry conidia to soil produced 40% mortality in A. obliqua adults. However, mortality was lower (21.2%) on evaluation under field cage conditions. According to the multiple decrement life table analysis, the probability of death of A. obliqua was 88% when C. haywardi parasitoids and B. bassiana conidia were used in conjunction, 89% when only C. haywardi parasitoids were released and 23% when only B. bassiana conidia were applied. These results demonstrate that no synergistic, additive or antagonistic interaction took place with the simultaneous use of these natural enemies, since the presence of B. bassiana had no effect on the C. haywardi parasitism. These results indicate that the parasitoid is a better natural enemy for the control of A. obliqua, and show that, although the two biological control agents can be used simultaneously, their joint application will not produce increased control.

Insect symbionts in food webs

Recent research has shown that the bacterial endosymbionts of insects are abundant and diverse, and that they have numerous different effects on their hosts' biology. Here we explore how insect endosymbionts might affect the structure and dynamics of insect communities. Using the obligate and facultative symbionts of aphids as an example, we find that there are multiple ways that symbiont presence might affect food web structure. Many symbionts are now known to help their hosts escape or resist natural enemy attack, and others can allow their hosts to withstand abiotic stress or affect host plant use. In addition to the direct effect of symbionts on aphid phenotypes there may be indirect effects mediated through trophic and non-trophic community interactions. We believe that by using data from barcoding studies to identify bacterial symbionts, this extra, microbial dimension to insect food webs can be better elucidated.

This article is part of the themed issue ‘From DNA barcodes to biomes’.

Olfactory response of Phytoseiulus persimilis (Acari: Phytoseiidae) to untreated and Beauveria bassiana-treated Tetranychus urticae (Acari: Tetranychidae)

Determination of attraction and avoidance behavior of predators is important in concomitant use of multiple natural enemies to control a pest. The olfactory response of the predatory mite Phytoseiulus persimilis was studied to odors related to Tetranychus urticae adults infected by Beauveria bassiana DEBI008 in 0, 24, 48 and 72 h intervals, both in absence and in presence of plants. In plant-present experiments, P. persimilis attraction was neither towards adults of T. urticae infected by 0.02 % Tween 80 (as control), nor to the ones infected by B. bassiana for 0 or 24 h, whereas significant attraction towards the control was observed when tested against T. urticae infected by B. bassiana for 48 or 72 h. In absence of plants, P. persimilis displayed significant avoidance of T. urticae infected by B. bassiana for 48 or 72 h, when their alternative option was 0.02 % Tween 80-infected T. urticae adults. These results indicate that P. persimilis can recognize the presence of B. bassiana and that the predator avoids the fungus. This suggests that the two natural enemy species can be used together in biological control programmes.

Virulence of Verticillium lecanii (Z.) against cereal aphids; does timing of infection affect the performance of parasitoids and predators?

BACKGROUND

Entomopathogenic fungi such as Verticillium lecanii (Z.) (Mycotal(®)) are used for pest control as an alternative to chemical control. In this study, the effect of V. lecanii on cereal aphids is assessed. In addition, an investigation is carried out to determine whether the use of V. lecanii affects the performance of two natural enemies of aphids, the predator Harmonia axyridis (P.) and the parasitoid Aphidius colemani (V.), in no-choice experiments under laboratory conditions.

RESULTS

The number of Rhopalosiphum padi (L.) and Sitobion avenae (F.) killed was increased by increasing the concentration of V. lecanii. The timing of application of fungus to aphids affected the efficacy of other biocontrol agents, a parasitoid and a predator. Parasitation by A. colemani (V.) in both cereal aphids (S. avenae and R. padi) was not affected by V. lecanii when aphids were first treated with V. lecanii and then exposed to A. colemani. The emergence of adults from parasitised mummies was, however, lower in infected aphids than in uninfected aphids when the aphids were first exposed to the parasitoids and then treated with fungus. The female sex ratio in the emerging adults was lower in V. lecanii-treated aphids in both species. When aphids were first treated with V. lecanii, 72 h before predation, fewer aphids of both species were consumed by H. axyridis (P.).

CONCLUSION

Use of entomopathogenic fungus as a biological control agent could be a complementary strategy in an integrated pest management programme against cereal aphids, but it can reduce the efficiency of other biocontrol agents (parasitoids and predators) when applied simultaneously.

Mesocosm experiments to assess the transmission of Pandora neoaphidis within simple and mixed field margins and over the crop-margin interface

Although considerable research on the development of agri-environment schemes has focussed on the value of managed field margins as reservoirs for arthropod natural enemies, their potential as reservoirs of entomopathogenic fungi has received less attention. Whether field margins that are most beneficial for arthropod natural enemies are the same as those for entomopathogenic fungi is unknown. Here, within glasshouse mesocosms, we assessed the reproductive success of the aphid-specific entomopathogenic fungus Pandora neoaphidis on aphids in a 'simple margin' containing one plant species and on the same species of aphid in a 'mixed margin' containing seven plant species. These assessments were done in the presence of Aphidius ervi, a hymenopteran parasitoid of aphids regarded as being a key species to conserve in agri-environment schemes in the UK. When only the plants initially infested with aphids were assessed, transmission of P. neoaphidis was significantly greater (p<0.001) in the mixed margin as was parasitisation by A. ervi (p<0.05). However, when all of the plants in the mesocosms were assessed, transmission of P. neoaphidis remained greater in the mixed margin (p<0.05) whereas parasitisation by A. ervi was greater in the simple margin (p<0.05). This difference may be due to aphid dispersal which was greater in the simple margin thereby benefitting the actively foraging parasitoid whereas clustering of aphids in the mixed margin benefited the passively dispersed fungus. In a second mesocosm experiment, the movement of P. neoaphidis over the crop-margin interface was similar to that of A. ervi despite the fungus only being passively dispersed in contrast to the actively foraging parasitoid. The results presented here indicate that, although the optimal plant composition of field margins may differ for P. neoaphidis and A. ervi, both species can co-exist and reproduce in field margins and will move over the crop-margin interface. Managed field margins that benefit both key arthropod and key microbial enemies have potential for enhancing pest control in associated crops.