For 20 years, ecologists have been gathering evidence in support of the hypothesis that plants can use insect natural enemies such as predators and parasitoids as bodyguards to protect themselves from herbivory, but entomopathogens have escaped this consideration. We extend the bodyguard hypothesis to ask whether plants can use entomopathogens as bodyguards. We first discuss the evolutionary context of such tritrophic interactions and then categorize possible mechanisms as: (1) maintaining a population of bodyguards on the plant surface, (2) increasing contact rates between insect host and pathogen and (3) increasing the susceptibility of the host. We explore these mechanisms further, examining published studies for evidence for the hypothesis. We then discuss potential costs to the plant of promoting pathogens as bodyguards which may include a reduction in the efficiency of other``bodyguard'' species, the incidental promotion of plant pathogens and the risk of entomopathogens developing phytopathogenicity. Aside from our intention to stimulate the testing of the bodyguard hypothesis with entompathogens and to provide a conceptual framework for this, we hope to bring evolutionary ecology and insect pathology closer together.

Successful control of greenhouse whitefly may be achieved by complementary activity of the parasitoid Encarsia formosa and the fungus Aschersonia aleyrodis. One way to obtain an additive mortality effect of both entomopathogen and parasitoid would be achieved by the selection of healthy hosts by the parasitoid and rejection of fungus‐infected hosts. Third and fourth instar larvae of Trialeurodes vaporariorum which had been treated with a spore suspension of A. aleyrodis 0, 4, 7, 10 or 14 days beforehand, were presented to female parasitoids. The parasitoids adopted the oviposition posture on untreated hosts as well as on treated hosts, irrespective of the different stages of infection in the hosts. However, significantly more hosts were parasitized by E. formosa in the control treatment than in the fungal treatment. The parasitoids offered treated hosts, showed rejection behaviour after probing on hosts showing detectable signs of infection (containing hyphal bodies or mycelium in the haemolymph). For instance, when hosts were offered seven days after spore treatment, the parasitoids showed an oviposition posture on a total of 83 (95.4%) out of 87 infected larvae, but laid only 4 eggs (4.6%). In contrast, on 48 (94.1%) out of 51 noninfected (or showing no detectable signs of infection) hosts an oviposition posture was adopted and 40 eggs (78.4%) were found after dissection. When infected hosts were encountered the oviposition posture lasted less than 1′40″ while rejection of non‐infected hosts occurred after more than 1′40″. Other experiments were carried out offering treated hosts for 24 h to the parasitoids. The hosts were dissected afterwards. Again, significantly more eggs were laid in the non‐infected hosts. When hosts were parasitized shortly after fungal spore treatment they were colonized by the fungus and the parasitoids did not develop. Transmission of the entomopathogen after probing infected hosts was observed to a limited extent. In conclusion, A. aleyrodis and E. formosa can be used together in a glasshouse situation. The parasitoid will be most effective when introduced more than seven days after application of A. aleyrodis, because from that time onwards it is able to detect and reject fungus‐infected hosts.

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Some of the factors considering parasitoid efficiency were investigated. Attention was focused on the effect of host density on parasitism and the appearance of host selection in the Encarsia formosa‐Trialeurodes vaporariorum relationship. Functional responses were determined by offering E. formosa different densities of either first, second, third of fourth whitefly larval instars during a fixed time period. The relationship equated to a type II functional response. No correlation was found between the numbers of ovipositor punctures on the larvae and the numbers of parasitoid eggs, therefore, the numbers of parasitized whitefly larvae were recorded by dissection for parasitoid eggs. A statistical model was developed to analyse the results from experiments in which mixtures of different instars were offered to E. formosa. Because the parasitoid searches at random once landed on the leaf, host encounters are dependent on the numbers of hosts and their size. Host preference also plays an important role. A significant preference of E. formosa for 2nd instar larvae to 1st, 3rd to 1st and 4th to 1st, and for 3rd + 4th to 2nd could be concluded from a statistical analysis of the data. Zusammenfassung Funktionelle Reaktion und Wirtsbevorzugung von Encarsia formosa Gahan (Hym., Aphelinidae), ein Parasit von Trialeurodes vaporariorum (Westwood) (Hom., Aleyrodidae) in Gewächshäusern Die Arbeit behandelt einerseits den Einfluß der Wirtsdichte auf die Parasitierung durch E. formosa und andererseits die Wirtswahl in der E. formosa/T. vaporariorum‐Beziehung. Durch das Angebot unterschiedlich vieler Wirtslarven jeweils des 1., 2., 3. oder 4. Larvenstadiums wurden funktionelle Reaktionskurven bestimmt. Die Beziehung zwischen der Anzahl angebotener und davon parasitierter Larven entsprach einer funktionellen Reaktion des Typs II. Es bestand keine Korrelation zwischen der Zahl der Ovipositor‐Narben auf den Larven und der von E. formosa abgelegten Eier. Es wurden daher die Larven seziert, um die Zahl der abgelegten Eier zu bestimmen. Die Ergebnisse der Wirtswahluntersuchungen wurden mit einem hierfür entwickelten statistischen Modell ausgewertet. E. formosa wurden Mischungen der verschiedenen Larvenstadien der Weißen Fliege angeboten. Da der Parasit den Wirt rein zufällig sucht, wird die Begegnung mit dem Wirt durch Wirtsdichte und Wirtsgröße beeinflußt. Die statistische Analyse der Ergebnisse zeigt deutlich, daß E. formosa das 2. Larvenstadium dem ersten, das dritte dem ersten und das vierte dem ersten vorzieht. Weiterhin ergab sich bei Mischung eine Bevorzugung des 3. und 4. Larvenstadiums vor dem 2. Larvenstadium.