Invasive species (14): The eucalyptus weevil, the bad good?

The eucalyptus weevil is an atypical invasive species . When we refer to a species as invasive, we tend to think that it causes damage to the invaded ecosystem. But this is not the case with Gonipterus scutellatus , the eucalyptus defoliator weevil, which feeds only on an invasive plant (eucalyptus) and produces a very serious impact in those places where it proliferates.

Life cycle of the eucalyptus weevil

Adult beetles are up to a centimeter long by 4-5mm wide. Their color is gray or reddish and they have a characteristic elongated “snout” with two large black eyes. They are harmless to humans, but if they catch their feet on the skin, it takes some effort to remove them. This is more of a curiosity than a nuisance.

Invasive species
Invasive species

The beetles lay their eggs in the lower part of the eucalyptus leaves, from where the larvae emerge. These feed on the shoots of the stems, as well as on the leaves, in which they leave characteristic black grooves. The adults also feed on the leaves, particularly on the outside.

They cause the leaves of the plant to fall, with the consequent impact on the growth of the tree . A side effect of the predation that this insect causes on eucalyptus trees is that the wood becomes harder.

Doubtful classification as a pest

The only negative impact that this species causes is in the profits that loggers obtain from the sale of eucalyptus. As it does not feed on any other plant species, its impact on the habitats where it is introduced is limited to eucalyptus plantations, which generally do affect ecosystems (being fast-growing trees, which dry out the soil, favor forest fires and they have toxic leaves, for which the native fauna cannot feed on them).

Invasive species
Invasive species

In Spain there are several species of eucalyptus, all native to Australia, of which the most widespread is Eucalyptus globulus . The eucalyptus is not lawfully classified as invasive species , despite be, as this will cause that could not be economically exploit its timber. The economic interests behind this sector, greater than those produced by any other invasive species, prevent eucalyptus from being classified as such.

Pressure from groups such as the Spanish paper employers’ association ( ASPAPEL ) led in the 1990s to measures against the proliferation of the eucalyptus weevil. This species has predators in its natural habitat, in Australia, but it does not have them in Spain. One of the measures taken was biological control using a species of parasitic wasp, Anaphes nitens .

These types of species are called parasitoids since their host dies during the life cycle of the insect.

This wasp, following the typical life cycle of parasitic wasps, lays its eggs in the larvae of the eucalyptus weevil. The larvae grow inside the weevil larvae, devouring it but delaying its death for as long as possible. Finally, they undergo their metamorphosis and emerge from the corpse of the weevil larva, transformed into adult wasps.

This wasp had been used previously in countries such as Madagascar and Ireland, where it gave good results and it was found that it did not interact with other species apart from the eucalyptus weevil. Its release was approved in Spain, where it maintains the populations of Gonipterus scutellatus at a stable level .

Another measure that was studied was the use of an insecticide whose active principle is flufenoxuron, which is being banned in the European Union . In 2012, a plan by the Xunta de Galicia was released to fumigate the forests with this compound using light aircraft, but the intense wave of criticism that occurred prevented the plan from going ahead.

The insecticide not only affected the eucalyptus weevil, but all types of arthropods that undergo metamorphosis during their life cycle , including bees and the parasitic wasp itself Anaphes nitens . For now, the only measure that has been taken against the insect is to use the wasp as a biological control, and it seems effective.

It is clear that economic interests are important when planning environmental conservation, but we must ask ourselves to what extent we are willing to sacrifice our great natural heritage for the benefit of certain sectors.

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