LIFE HISTORY OF THE APHID HYPERPARASITOID ASAPHES VULGARIS WALKER (PTEROMALIDAE): POSSIBLE CONSEQUENCES ON THE EFFICACY OF THE PRIMARY PARASITOID APHIDIUS NIGRIPES ASHMEAD (APHIDIIDAE)

Aspects of the life history of the aphid hyperparasitoid Asaphes vulgaris Walker, when reared on Aphidius nigripes Ashmead, were investigated under laboratory conditions. Longevity was significantly related to temperature and to the sex of adults. Female life span was about 4 months at 15 °C compared with 46 days at 25 °C; under the same conditions male longevity was 66 and 19 days, respectively. Females started to oviposit in aphid mummies upon eclosion and lifetime fecundity was high, reaching 1433 offspring. The sex ratio (proportion of males) was low early in reproductive life but rose sharply after several weeks. The implications of these results on the efficacy of A. nigripes in the potato agrosystem are discussed. It is suggested that high hyperparasitoid fecundity and longevity might play a role in reducing the impact of the primary parasitoid.

Honeydew as a source of host-searching kairomones for the aphid parasitoid Aphidius nigripes (Hymenoptera: Aphidiidae)

The parasitoid Aphidius nigripes spent more time searching on potato plants previously infested with aphids than on fresh plants, suggesting the presence of searching stimulants on infested plants. Water extracts of aphid honeydew applied to the surface of foliage or to filter paper discs increased searching time and affected the locomotory behavior of parasitoids. When crossing a honeydew-treated area on filter paper, female parasitoids exhibited arrestment, antennal examination, abdominal protraction, reduced walking speed, and increased turning. Upon losing contact with honeydew-contaminated substrate, parasitoids often exhibited a klinotactic response which drove them back to the treated area. The time spent by individual parasitoids within a honeydew-treated area on filter paper decreased with successive visits, but the initial response level was restored when the insect was prevented from contacting honeydew for 1 h. The retention response exhibited by parasitoids contacting a honeydew-treated area increased with honeydew concentration, but levelled off at high concentrations. Naive parasitoids were more responsive to the retention effect of honeydew than experienced individuals.

Host discrimination in the aphid parasitoid Aphidius nigripes

Females of the solitary aphid parasitoid Aphidius nigripes do not consistently avoid superparasitizing their hosts, the potato aphid Macrosiphum euphorbiae. Tests were conducted in which two attacks separated by a time interval ranging from 1 to 24 h were allowed per host. No evidence of host discrimination was found when the interval was <2 h. For longer intervals, host discrimination was an increasing function of the length of the interval up to 24 h after the initial attack. Various mathematical models, permitting time-related rejection of parasitized hosts, were used to simulate the parasitoids–host distributions following two attacks. The best fitting models also had to incorporate time-dependent discrimination against hosts that had been attacked but not successfully parasitized. The results are discussed with respect to possible mechanisms of host discrimination in A. nigripes and its significance in intraspecific competition.

THE EFFECT OF HOST DENSITY ON EGG DISTRIBUTION BY THE SOLITARY PARASITOID APHIDIUS NIGRIPES (HYMENOPTERA: APHIDIIDAE)

Egg distribution by the solitary parasitoid Aphidius nigripes was studied experimentally at 14 densities of the host aphid, Macrosiphum euphorbiae. The number of hosts parasitized increased with host density, the functional response being of Holling's type II. However the average number of eggs/female was not significantly affected by host density, and consequently superparasitism increased with decreasing host density. When egg distributions were tested against random models, higher than predicted numbers of both unparasitized and superparasitized hosts were observed at all densities tested. Further analysis assuming random search suggested that female A. nigripes tend to deposit several eggs per host while experiencing a low rate of encounter with hosts. The ecological significance of this behaviour is examined.