There was no significant difference in the number of males preforming homosexual (n = 13 and 16, for mated and virgin males, respectively) and mating competition behaviours (7 mated and 9 virgin males, respectively) between mated and
virgin males (χ2 = 1.433 and 0.001 for homosexual and mating competition
behaviours, respectively; df = 1,110; P > 0.05). After mating occurred, no mated or virgin males could separate the mating couples.
Virgin males spent a slightly longer period in homosexual behaviour and mating competition but no significant difference was found when compared with
mated males (ANOVA: F = 0.097 and df = 1,17 for homosexual period; F = 1.519 and df = 1,14 for competition period; P > 0.05) (Table 5.2).
Table 5.2 The mean (± SE) period (seconds) of homosexual and competition
behaviourperformed by A. ervi males.
Homosexual Competition
Mated male 62.56 ± 15.56 a 52.25 ± 10.51 a
Virgin male 69.67 ± 16.65 a 69.13 ± 8.78 a
Means (± SE) followed by the same letters in columns are not significantly different
(P > 0.05).
5.3.5 Discussion
Behavioural observations indicate that males appear to be more active than
females in the mating behavioural sequence of A. ervi. This may be because a virgin
female can reproduce without mating but the success of male reproduction absolutely
relies on, and is mediated by females (Quicke 1997). Moreover, as detected in A.
nigripes (McNeil and Brodeur 1995), A. ervi females release a sex pheromone that triggers the male behavioural sequence of wing fanning, approaching and mounting (Battaglia et al. 2002; McClure et al. 2007). My results show that the courtship behaviour by males (i.e. wing fanning and approaching) is non-random, indicating
that in A. ervi, females produce signals and males do the searching.
Tactile stimulation may play an important role in mating success of A. ervi.
The male moved his antennae rapidly from side to side and contacted those of the female. This behaviour seemed to have quietened the female. Previous reports show that males of many hymenopteran wasps have glands in their antennae (Isidoro and
Bin 1995; Isidoro et al. 1996; Battaglia et al. 2002). Males of A. ervi emit secretion
from their antennae when they are exposed to virgin females; the secretion functions specifically as a contact pheromone and the antennae play a pivotal role in sex recognition and partner acceptance (Battaglia et al. 2002).
In theory, female choice is clearly an important driving force in sexual selection in animals, and a few examples of adaptive choice are known in natural
enemies, particularly fly predators such as the black-tipped hanging fly Hylobittacus
apicalis (Hagen) (Thornhill 1988) and scorpion fly Panorpa vulgaris Imhoff and Labram (Thornhill and Sauer 1991). However, in parasitoids there is no evidence that females exhibit any active choice (Hardy et al. 2005). A virgin parasitoid female will usually mate with the first conspecific male encountered, for example, in a gregarious
parasitoid N. vitripennis (van den Assem et al. 1980). In the present study, A. ervi
females usually mated with the first mounting males regardless of their virginity. This may be because of the difficulty of finding another suitable partner (van den Assem et
al. 1986). As found in other Aphidius species (Mackauer 1969), A. ervi females
usually became receptive after several approachings by males. It is still not clear whether the females have means to assess their partners’ quality during this period.
Obvious male aggression behaviour was not observed in A. ervi, but male-
male interactive behaviour such as male courtship display and mating attempt to the rival was common when males encountered each other. Homosexual mounting between males is found in a number of insect species including aphidiids (Starý 1970; Thornhill and Alcock 1983; van den Assem 1986; Kaneshiro and Giddings 1987; Gillespie 1991; Wang et al. 1996; Serrano et al. 2000; Cheng et al. 2003; Switzer et al. 2004). It is expected that mated males have learnt from the previous mating in discerning the potential mates and rivals and avoid homosexual mounting; however, results indicate the frequency of homosexual mounting between mated and virgin
males was similar. Homosexual interactions between males in A. ervi may be elicited
by the female sex pheromone, as reported in another aphidiid, Praon aguti Smith
(Sekhar 1957; Starý 1970). Visual cues may also result in male mistake in courtship in some insects (Thornhill and Alcock 1983; Wang et al. 1996), including some hymenopteran parasitoids (van den Assem 1986). According to Thornhill and Alcock
(1983), althought males are at the risk of making some mistakes, mounting in A. ervi
may be an important process in discriminating a mate.
Male mating experience is a factor that affects mating success in A. ervi.
Results of hourly mating experiments (Section 4.5) indicate that females can produce similar number of female offspring when mated with virgin males or once-mated
males, thus females are not expected to have preference between virgin and once- mated males for mating. In the present study, virgin males invested more energy in courtship (i.e. wings fanning and approaching) for courting females than mated males. Mated males responded to females more quickly and performed fewer approaches to females before mounting, and achieved more matings. These suggest that mated males have learnt from the previous mating, so that they respond to females more quickly and perform better courtships than naïve males. It has been reported that in some hymenopteran parasitoids (Perez-Lachaud and Campan 1995; Fischer and King 2005)
and other insects (Schwartz 1991; Cook 1995) male mating experience enhances
mating success. However, McClure et al. (2007) indicated that under no-choice
situation, A. ervi males that have previously mated have longer pre-courtship period
but have similar mating success when compared to virgin males of similar age. Thus,
it is suggested that under competitive situation, mated A. ervi males may sense the
presence of rivals, court females quickly and then achieve better mating success.