In general, the foraging behaviour of hymenopteran species is divided into three mechanistic steps: (1) host habitat location, (2) host location and (3) host acceptance (Doutt 1959; Vinson 1976). Chemical and physical cues appear to play a major role at almost every level of the foraging behaviour (Vinson 1984; Schmidt 1991; Vet and Dicke 1992; Turlings et al. 1993; Godfray 1994; Quicke 1997). Success of host searching depends upon the execution of appropriate behavioural responses to an array of cues which are available during foraging.
Host habitat location. Plant volatiles emanating from the host’s food and host odors have been shown to be important cues in host habitat location at long distances
for a number of hymenopteran parasitoids. In a wind tunnel bioassay, naïve A. ervi
females responded poorly to undamaged bean plants and to pea aphid isolated from the plant, but they showed strong oriented flight responses to aphid-infested plants and to aphid-damaged plants from which the aphids had been removed (Guerrieri et al. 1993; Du et al. 1996). When offered a choice, more parasitoids flew toward and landed on the host-infested or host-damaged plants than on undamaged plants (Du et
al. 1996). These results suggest that A. ervi females use host-induced plant volatiles as
host habitat location cues.
The compounds of volatile extracts from pea aphid infested plants that could be involved in parasitoid attraction, are identified as 6-methy1-5-hepten-2-one,
linalool, (E)-β-ocimene, (Z)-3-hexeny acetate, (Z)-3-hexen-1-ol, and (E)-β-farnesene,
with A. ervi appearing to use the more specific, 6-methy1-5-hepten-2-one, which is
not induced in plants infested by the non-hosts, such as black bean aphid, Aphis fabae
Scopoli (Du et al. 1998). Thus, 6-methy1-5-hepten-2-one is potentially one of the
volatile components that allow A. ervi to distinguish between plants infested with
hosts and non-hosts (Du et al. 1998), and its ability has been demonstrated in the wind tunnel study (Du et al. 1996).
The parasitoid experience enhances searching ability in at least 20 species
(Turlings et al. 1993). In the wind tunnel study, responses of A. ervi females to
volatiles emanating from undamaged, host-infested and damaged or host-damaged plants are enhanced by previously exposing the females to the plants infested by aphids (Du et al. 1997; Guerrieri et al. 1997). Thus the plasticity of parasitoid foraging
behaviour may provide ways of enhancing the efficiency of A. ervi in pest
management (Guerrieri et al. 1997).
Moreover, long range attraction of A. ervi toward monochromatic light in the
green region of the spectrum (514 nm) has been reported (Goff and Nault 1984), which suggests that this parasitoid employs a visual response to green vegetation during the host habitat location phase of host selection. The pea aphid sex pheromones as a potential host habitat location cues have also been demonstrated for
Host location and acceptance. In a short range, host location and acceptance by A. ervi are influenced by the physical cues. A. ervi females show strong attack responses toward green pea aphid even without physical contact, indicating the use of
visual cues during host location (Battaglia et al. 1995). A. ervi females react to yellow
pigment with repeated oviposition attack responses, but they do not react to green pigment (Battaglia et al. 2000). The spectrum of reflected light from the yellow pigments is very similar to that from the ‘green’ natural pea aphid, with a high proportion of the total radiation energy being emitted in the yellow-orange wavebands (580~660 nm) (Battaglia et al. 2000).
The host location and acceptance by A. ervi are also regulated by
semiochemical cues. It has been shown that the cornicle secretion, cuticula and honeydew of pea aphid act as contact kairomones and elicit an intense oviposition
attack response by A. ervi females, which appears to be innate (Battaglia et al. 1993;
Pennacchio et al. 1994; Du et al. 1997). A. ervi females do not attack wet pea aphid
that are washed previously with water, but one hour later this phenomenon disappears and A. ervi females attack washed aphids to the same degree as dry ones (Weinbrenner and Volkl 2002), confirming the chemical cues on the cuticula of aphids as contact kairomones.
In the field, A. ervi females may encounter parasitised and unparasitised hosts
and the parasitised hosts may have been attacked by conspecific or heterospecific parasitoids. Host discrimination (i.e. the ability to distinguish parasitised and unparasitised hosts) is a common phenomenon among Hymenopteran parasitoids (van
Lenteren 1981). In A. ervi-blue-green lucerne aphid system, newly parasitised hosts
appear to be susceptible for superparasitism but parasitoids tend to avoid superparasitism six hours after the hosts are attacked (Micha et al. 1992). This
suggests that A. ervi females do not mark their hosts with an oviposition-deterring
pheromone to limit short term superparasitism, and the trend to avoid superparastism at longer intervals is probably attributed to changes occurring in the host as a response by the aphid to parasitism or to changes induced by the presence of a parasitoid egg
within the host (Micha et al. 1992). A. ervi females generally avoid ovipositing in a
pea aphid which has been parasitised by another species, such as Aphelinus asychis
competing larvae by physical and possibly physiological suppression (Bai and Mackauer 1991).
2.7 Reproductive Biology of Aphidiinae