The apple blossom weevil is eaten by small birds such as tits but the most important natural enemies are two species of parasitic wasp. Because small numbers of apple blossom weevil can be tolerated economically, these parasitic wasps might be important natural enemies.
Parasitic wasps
The ichneumon parasitic wasp Scambus pomorum is a common parasite of apple blossom weevil.
- The adult female parasitic wasp pierces the capped blossom with her ovipositor, stings the apple blossom weevil larva and injects a paralysing poison. She then deposits a transparent, elongate egg onto the larva.
- The egg hatches in a few days and the parasite larva attaches itself to the dorsal side of the host larva with its mouth parts immersed within the tissue of the latter. The fully-grown larvae are elongate and fusiform in shape, narrowing towards the extremities.
- There are four larval stages. Eight to ten days after hatching, the fully grown parasite larva leaves the remains of the host to spin a thin silken cocoon within the cavity of the capped blossom.
- Pupation is completed in 4-5 days. The pupa turns black and the adult wasp emerges about 11 days later, about 2 weeks later than adults of the apple blossom weevil.
- Up to 50% parasitism has been recorded. Scambus pomorum does not have a second generation. After emergence, the females appear to be in a state of reproductive diapause.
- The adults feed on a wide range of types of larvae (moths, flies, sawflies) throughout the summer and overwinter.
The other parasitic wasp is the braconid Syrrhizius delusorius which parasitises the adult apple blossom weevil. This parasite is believed to have 2 generations a year.
- It overwinters as a small (about 1.2 mm long) larva in the abdomen of the adult weevil.
- In May, the full-grown larva leaves the host through a small hole it makes in the exoskeleton of its host.
- The emerging larva is very active and spins a cocoon within 24 hours within which it pupates.
- The adult wasp emerges a few weeks later in synchrony with the emergence of adult weevils.
- The female parasite climbs onto the back of the weevil and stings into the abdomen under the wing cases.
- Females become excited when offered a capped blossom, even when empty, indicating the way the host is located.
Biological control
No specific biological control measures have been developed. The natural enemies of apple blossom weevil, especially the parasitic wasps, should be fostered by avoiding the use of harmful insecticides (see above). Nesting boxes could be provided for tits which might help reduce populations.
Biotechnological control
It has been shown that the weevil is strongly attracted by host plant volatiles produced by bursting apple buds. Some of the terpenoid chemical compounds involved have been identified. It might be possible to exploit these for monitoring or control of the pest but such methods have not been investigated or developed as yet. Work is in progress at East Malling Research to identify a sex pheromone of apple blossom weevil