Host-marking pheromones

Host marking pheromones are important in many species of parasitic hymen-optera, because they ensure that a female parasitoid focuses on non-parasitized hosts. This, in turn, ensures a more effective use of limited host resources. Marking pheromones can be internal (injected into the host at the time of oviposition) or external (applied to the host during inspection and/or ovipo-sition). The internal markers can be detected by sensory hairs on the parasitoid ovipositor [11]. The internal markers often also delay the development of the host. 

Oviposition deterring pheromone (host marking pheromones). 

All of the suborder Symphyta and many species in the superfamily Aculeata in the suborder Apocrita are solitary insects. Although not requiring the complex semiochemistry of parasitic or social insects, solitary insects employ pheromones for mating, territorial marking, and host marking. Unfortunately, very few of these have been chemically identified. The pheromones of sawflies and seed wasps were extensively reviewed in 1999 [ 14]. The semiochemicals recently identified in solitary hymenoptera, discussed below, are summarized in Table 2 and Fig. 1. 

Unlike parasitoids of other insect orders that have host-seeking larvae, most parasitic hymenoptera lay their eggs on, in, or very close to a host individual [11]. This requires the adult female to find a suitable host, often with the aid of chemical cues from host frass, pheromones, plant volatiles emitted upon host feeding or egg-deposition, silk, honeydew and other secretions. She may then chemically mark the host following oviposition to reduce superparasitism by herself or intra- and inter-specific insects [11]. 

Tetratrophic interactions between a host plant, a phytophagous pest (primary host), a hymenopteran parasitoid or symbiont (secondary host) and a hymenopteran hyperparasitoid (which parasitizes the secondary host) are of considerable importance, because hyperparasitism can significantly reduce populations of economically beneficial parasitoids [11]. Hyperparasitoids use host-marking (=spacing) pheromones, sex pheromones [12], and host-detection cues [42], but they also show additional chemically mediated interactions with the other partners. These include detection of the primary host s secretions by the hyperparasitoid [43], detection of plant volatiles by the hyperparasitoid [44], and detection of the hyperparasitoid s secretions by the primary host [45] or by the secondary host. The latter causes the secondary host to avoid locations where the hyperparasitoid is foraging [46]. 

The oily secretions produced by the Dufour s gland have been shown to function as host marking compounds and sex pheromones... 

Host sex pheromones can serve as cues to host location for several parasitoid species (Kennedy, 1979 Sternlicht, 1973). Prokopy and Webster (1978) found that the marking pheromone of Rhagoletis pomonella stimulates oviposition probing of Opius lectus. 

Parasitoid marking pheromones, according to van Lenteren (1981) (i) prevent multiple ovipositions into the same host and consequent egg wastage, (ii) prevent host wastage because superparasitized hosts often die, resulting in death of the parasitoids, (iii) save time, particularly if the process of attack and oviposition are lengthy and (iv) initiate migration to more productive habitats when several marked hosts are encountered. 

In none of his experiments could Moeck demonstrate that primary attraction is used in host selection by the three principal species in the area Dendroctonus brevicomis (the western pine beetle), D. ponderosae (the mountain pine beetle) and Ips paraconfusus (the California five-spined engraver beetle). What Moeck s results did show, however, was that beetles landed indiscriminantly on healthy and stressed trees at about one beetle a day on each tree. Theoretically only one beetle is needed to initiate mass attack, since as soon as it releases pheromone, landing rates would increase markedly on that tree. In Moeck s study, landing rates on trees that became attacked by D. brevicomis increased to up to 800 beetles a day. 

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