Comparative Effect of Wasp Density in Unisexual and Bisexual Strains of Lysiphlebus fabarum, on Superparasitism in the Black Bean Aphid, Aphis fabae

Lysiphlebus fabarum Marshall (Hymenoptera: Braconidae: Aphidiinae) is a common aphidiine parasitoid in central Europe, where it attacks more than 70 species of aphids. Although both unisexual (thelytokous) and bisexual (arrhenotokous) strains of L. fabarum have been reported in Iran, the former appear to be more widely distributed. Aphid control in Iran relies heavily on pesticides in both field and greenhouse settings, whereas any availability of new biological control agents such as L. fabarum would likely aid in reducing pesticide use. One of the most important aspects of implementing a parasitoid as a biological control agent is to determine the best density of parasitoid in the host access, in mass rearing. Parasitoid wasps display a wide array of strategies to dispose of host resources. Superparasitism is parasitism of a host by parasitoids of the same species. In solitary parasitoids, superparasitism almost occurs when hosts are limited or parasitoid density in a patch is high. Acceptance or rejection of a host usually depends on a female parasitoid’s ability to distinguish parasitized and unparasitized hosts. Previously, superparasitism was thought to be a maladaptive strategy, wasteful of both eggs and time, while it is now recognized as adaptive in a number of situations, e.g. under conditions of low host availability, as a mechanism for overwhelming the immune response of hosts.
In the present study, stock colonies of the unisexual and bisexual strains of the parasitoid were established from materials collected from Zanjan province and Khuzestan province fields, respectively. Both populations of the parasitoid were collected from parasitized black bean aphids feeding on the broad bean, Vicia fabae L. A stock colony of black bean aphids was established with material collected from bean fields in Khuzestan province. The synchronous cohorts of both strains of L. fabarum were produced at 21±1 ºC, 50–60% r.h., and L14:D10 photoperiod, and then introduced (one day old) separately into 40 2nd instar nymphs in different densities (1, 5, 10, 15 and 30). The females were removed after 24 hours and parasitized aphids were dissected three days after parasitism to determine the number of wasp larvae in each host. As well as, in another experiment the super parasitized aphids were dissected in three consecutive days (3.5, 4.5 and 5.5) to determine the process of competitors’ elimination.
According to results, in both strains, wasp densities affected on parasitism rates. In the bisexual strain, females parasitized more aphids in 20 and 30 densities treatments than one density treatment, while this rate did not differ significantly between 1, 5, 10 and 15 densities treatments. In the unisexual strain, single females significantly parasitized fewer hosts than other treatments, but there was no difference between other densities treatments. When in each density, percentage parasitism was compared between two strains, in the density of one female, bisexual females parasitized significantly higher than unisexual ones, in contrast to the density of 30, which significantly more larvae were counted in hosts of the unisexual strain than bisexual ones. In other densities, there was no significant difference between two strains. In the second experiment, which the rate of superparasitism was compared in different densities, in bisexual strain, the rate of superparasitism significantly was lower in the density of one female than other treatments. The number of larvae did not differ significantly between other densities treatments. In the unisexual strain, the rate of superparasitism significantly was lower in the density of one female than other treatments. The most rate of superparasitism was observed in 20 and 30 density treatments. When in each density, the rate of superparasitism was compared between both strains, in 1, 5 and 10 densities, bisexual females superparasitism significantly higher than unisexual ones, in contrast to the density of 30 females, which significantly more larvae were counted in hosts of the unisexual strain than the hosts of bisexual ones. In association with the process of competitors’ elimination, bisexual strain larvae eliminated rival larvae over time (in three consecutive days, including days 3.5, 4.5 and 5.5), as the numbers of larvae were decreased in densities of 5, 20 and 30 females. While in unisexual strain, elimination of rival larvae was not observed over time.
Wasp densities affected by parasitism rates and the rate of superparasitism in both strains. In mass rearing, the best density of wasps can be defined as the density that most available hosts are parasitized with a minimum of superparasitism, because superparasitism can depredate the time and eggs of female foragers. The results showed that densities of one and 5 females (for 40 individuals of 2nd instar nymphs) can be suggested as the proper densities of females in bisexual and unisexual strains, respectively. The results of this study can be useful in mass rearing and release of both L. fabarum strains.