Community dynamics depends partly on the outcome of competition among species and may affect the management of herbivorous insect pests in tritrophic systems. The outcome of the competition between different parasitoid species sharing the same host is known to be affected by several factors (McBrein and Mackauer
1990; de Moraes et al.
1999; Collier and Hunter
2001; Collier et al.
2002; Pérez-Lachaud et al.
2002; de Moraes and Mescher
2005; Yamamoto et al.
2007). Of these, development time of the parasitoids from egg to adult stage, oviposition order between parasitoid species, development stage of the host when it is parasitized, and larval morphology of the different competitors are of importance (McBrein and Mackauer
1990; de Moraes et al.
1999; de Moraes and Mescher
2005; Yamamoto et al.
2007). In the present study, the egg-larval parasitoid
P. leucobasis was the first to oviposit in
M. vitrata eggs. Its development time from egg to cocoon was longer than that of the larval parasitoid
A. taragamae, which may explain why the latter outcompeted
P. leucobasis. In competition, the parasitization success of the egg-larval parasitoid
P. leucobasis was significantly reduced compared to that obtained for the control (in the absence of competition). When offered one- or two-days-old larvae, the percentage parasitism of the larval parasitoid
A. taragamae was not significantly affected by previous parasitization by
P. leucobasis in comparison with the control. However, the percentage parasitism for
A. taragamae in one-day-old larvae was significantly lower than that obtained with two-days-old larvae. This may be due to differential host suitability. These findings concur with those of Dannon et al. (
2010a) who reported that one-day-old larvae were less suitable to
A. taragamae development than two-days-old larvae. Differences observed in the development time of the two parasitoid species may affect the outcome of their competition. Because the development time of
P. leucobasis from egg to cocoon stage is longer than that of
A. taragamae (Table
1)
, we hypothesize that the eggs of the latter hatch earlier. Therefore, larvae of
A. taragamae might win the intrinsic competition with
P. leucobasis by developing different mechanisms such as physical or physiological attacks. Indeed, in competition, larvae of some parasitoids are known to outcompete their competitors through direct physical or physiological attacks (Fisher
1961; Vinson and Iwantsch
1980; Laing and Corrigan
1987; McBrein and Mackauer
1990; de Moraes et al.
1999; Muturi et al.
2006). The present study did not investigate the mechanisms involved in the intrinsic competition between
A. taragamae and
P. leucobasis. Results from these simplified experiments cannot be extrapolated to field conditions nor used to accurately predict the overall outcome of the competition between the two parasitoid species. Previous studies showed that the percentage parasitism of
A. taragamae increased with host density but the wasp was not able to parasitize all offered host larvae even in a small arena (Dannon et al.
2010a). Therefore, in heterogeneous and complex natural environments, a fraction of larvae may remain unparasitized. Thus, a spatio-temporal niche partitioning or dispersal-competition trade-off may lead to the coexistence of the two parasitoid species with optimum control of
M. vitrata through complementary action. Indeed, previous studies revealed that
P. leucobasis was found mostly on trees that are host plants of
M. vitrata such as
Pterocarpus santalinoides and
L. sericeus (Tamò et al.
2002; Arodokoun et al.
2006), while
A. taragamae was abundant on a shrub host plant
Sesbania cannabina in Taiwan (its origin) (Huang et al.
2003). This preference of
A. taragamae for
M. vitrata host plants with lower growth habit might lead to niche partitioning, thus contributing to the coexistence of
A. taragamae with
P. leucobasis.