Insect assistant number two—Ibalia

Ibalia was deliberately brought from Europe in 1950 and 1951, and was first liberated in 1954, after intensive study, in approved scientific fashion. It is now well-established almost everywhere Sirex occurs in New Zealand.

Ibalia is quite a bit smaller than Sirex or Rhyssa, its 16 mm being about half the length. The ovipositor does not trail behind, being retracted inside the body when not in use. Both sexes are distinctive in that they are flattened sideways like fleas. The female Ibalia detects by smell a recent drill-hole made by a female Sirex when egg-laying. The Ibalia uses the same hole, then pierces the Sirex egg, laying her egg inside the Sirex egg so that when the Sirex grub hatches it already has a parasite growing inside it. The Ibalia can lay her egg inside an already-hatched Sirex grub, but this only happens if the Sirex grub has not moved away from its hatching site; the Ibalia female has to use the Sirex female's drill-hole, as she has no timber-drilling apparatus herself.

The Ibalia larva grows inside the Sirex grub for several months, savouring the meal, and then bites its way out to eat the moribund remnants. It then pupates, and the adult after hatching chews its way out of the wood. Like Rhyssa, the Ibalia adults live on honeydew from aphids and scale insects. Ibalia and Rhyssa, with some other biological controls, keep the numbers of Sirex wasps down very satisfactorily, killing perhaps seventy percent of the pests. With good forest management, the infestation of the accidental immigrant Sirex can be said to be well under control.

Ibalia female egg-laying down a Sirex drill-hole.