Antagonistic Interactions between the African Weaver Ant Oecophylla longinoda and the Parasitoid Anagyrus pseudococci Potentially Limits Suppression of the Invasive Mealybug Rastrococcus iceryoides

Abstract

The ant Oecophylla longinoda Latreille forms a trophobiotic relationship with the invasive mealybug Rastrococus iceryoides Green and promotes the latter's infestations to unacceptable levels in the presence of their natural enemies. In this regard, the antagonistic interactions between the ant and the parasitoid Anagyrus pseudococci Girault were assessed under laboratory conditions. The percentage of parasitism of R. iceryoides by A. pseudococci was significantly higher on "ant-excluded" treatments (86.6% ± 1.27%) compared to "ant-tended" treatments (51.4% ± 4.13%). The low female-biased sex-ratio observed in the "ant-tended" treatment can be attributed to ants' interference during the oviposition phase, which disrupted parasitoids' ability to fertilize eggs. The mean foraging time, host handling time and number of successful oviposition in "ant-excluded" treatment were significantly higher compared to "ant-tended" treatments. When ant workers were allowed access to sterilized sand grains, mummified and unmummified R. iceryoides, they selectively removed the mummified mealybugs, indicating that they recognized the mummies as potential foods (1.2 ± 0.46 to 7.8 ± 1.17 mummies at 10 min intervals for 2 h). Percentage emergence from mummified R. iceryoides removed by the ants was significantly lower compared to emergence from mummies not exposed to ants. Although, host seeking parasitoids frequently evaded attacks, some were killed by the foraging ant workers (2.0 ± 0.38 to 6.0 ± 0.88 at 10 min intervals for 2 h). These results suggest for the first time that the presence of O. longinoda has a detrimental effect on the abundance, reproductive success and possibly oviposition strategy of female parasitoids, which might be a delimiting factor in field conditions if both natural enemies are to be recommended for use within the same agro-ecosystem.

Keywords: Anagyrus pseudococci; Oecophylla longinoda; Rastrococcus iceryoides; biotic interference; mummy mutilation; parasitoid mortality.

Figure 1

Oecophylla longinoda worker encounters with mummified mealybugs were always followed by an antennal examination (A) before transportation (B–D), indicating that they perceived the mummies as a potential food source.

Figure 2

Mean number (±standard error) of mummified Rastrococcus iceryoides removed and transported by the different Oecophylla longinoda ant groups (treatments) from the foraging cages at 10 min intervals over a 2 h observation period.

Figure 3

Mean (±standard error) percentage of Anagyrus pseudococci emerged from mummified Rastrococcus iceryoides removed by the different Oecophylla longinoda groups for a period of 2 h. Means with different letters are significantly different at p ≤ 0.05.

Figure 4

Mean (±standard error) number of Oecophylla longinoda on Rastrococcus iceryoides-infested butternuts and mean number of adult Anagyrus pseudococci killed by Oecophylla longinoda workers in the foraging cages at 10 min intervals over a 2 h observation period.

Figure 5

Oecophylla longinoda worker aggressively targets a fast running parasitoid (A); destabilizes it using its antenna (B) and finally seized the wasp with its mandibles (C).

Figure 6

The mean (±standard error) number of behavioural responses displayed by the parasitoid Anagyrus pseudococci to evade encounters and attacks by Oecophylla longinoda at 10 min intervals over a 2 h observation period.