on a seed alters female behavioural sequences, as well as the probability
of host acceptance (Messina and Dickinson, 1993; Parret al., 1996).Proximate Cues
Few studies have identified the cues used by herbivorous insects to detect
eggs or larvae on potential hosts (Hurteret al., 1987). Most investigations
have focused on chemical cues, which are called host-marking phero-
mones or oviposition-deterring pheromones. These cues may reside in the
eggs, larvae or larval faeces, but in many cases the egg-laying female
shows a distinct behaviour that results in pheromone deposition immedi-
ately after an egg is laid. For example, the female of the almond seed
wasp,Eurytoma amygdaliEnderlein, lays a single egg in the nucellar
tissue of a developing seed and then drags the tip of her abdomen on the
almond surface (Kouloussis and Katsoyannos, 1991). A series of experi-
ments demonstrated that this dragging behaviour, which has been well
studied in related fruit parasites, leads to the deposition of a water-soluble
marking pheromone.
The distinctive marking behaviour ofE. amygdalimay originate as a
means by which a female avoids reinfesting the same host. If egg-laying
females search for hosts over a relatively small area, the probability of
re-encountering a host can be high, and even a short-lived, water-soluble
pheromone can be effectively used by a female to avoid competition
among her own offspring. At the same time, a marking pheromone can
inhibit egg laying by later-arriving, conspecific females, whose offspring
would perform better in unoccupied hosts. Host-marking behaviour may
therefore evolve in a similar way to and under the same constraints as
other animal communication systems (Roitberg and Prokopy, 1987).
Marking pheromones deposited after oviposition are sometimes
secreted by specialized glands (Quiringet al., 1998; Fergusonet al.,
1999a). Most studies suggest that these compounds are perceived by
contact chemoreception (gustation) or short-range olfaction, so that the
female cannot detect whether a host is occupied without close inspection.
Ablations of sense organs and electrophysiological recordings from
particular sensillae have been used to identify the means of pheromone
perception (Fergusonet al., 1999b). Bioassays have shown that some
marking pheromones are quite stable; others lose effectiveness after only a
few hours. Theoretical analysis suggests that variation in pheromone
persistence will depend on whether the pheromone signal is exploited by
the parasite’s natural enemies (Hoffmeister and Roitberg, 1998)
Interest in the active components of marking pheromones has been
spurred by the potential use of these chemicals to control seed or fruit
parasites that are economic pests. If marking pheromones deter ovi-
position or cause females to disperse away from infested hosts, they might
be used to manipulate parasite behaviour in a way that reduces crop
damage. Unfortunately, identification of marking pheromones has beenHost Discrimination by Seed Parasites 69