Sunday, February 6, 2011

Weevils and caterpillars ready for action in the palms and pines

This is a pivotal time of year for the Red Palm Weevil and the Pine Processionary Moth. They are regarded by many as the top two most destructive and least endearing creatures in southern Portugal - but you've got to admire them for their tenacity and will to survive.

The success of the mindless Red Palm Weevil's spread from its native Southeast Asia all the way through the Middle East and right across southern Europe has been remarkable. It is in stark contrast to the hapless efforts to contain it by our own supposedly intelligent species, Homo sapiens. We should also mention in passing that while we blather on about the undesirable qualities of certain bugs, our own destructive instincts are immeasurably worse.

The first weevils to cross the Mediterranean were carried in infested adult palms imported from Egypt to the Costa del Sol in 1994. Although the weevil's traits were already internationally well-known, no importation restrictions were in place in Spain.

Two years went by before the Spanish government got around to imposing such restrictions. The relevant EU authorities dithered. Four years on, Spanish import restrictions were toned down. By then a lucrative trade in decorative palms was flourishing across open EU borders. The weevil was brought into the Algarve in 2007. It is now abundant and causing havoc.

What's happening in the weevil's world right now is that the adults are emerging from their winter hibernation. Females will start laying up to 200 eggs at a time in palm crevices, preferably in Canary Island palms.

The eggs will hatch within a few days and the larvae will start eating their way into the soft inside of the trunks. As these legless grubs devour their way deeper, they grow fatter and as long, if not longer than the adult weevils.

It is the ravenous appetite of the tunnelling grubs that causes the most damage to the trees. By the time obvious symptoms are showing, a tree is likely to be already heavily infested and its fate sealed.

On emerging bloated from the trunks and ready for the next stage in their short lives, the grubs construct oval cocoons of palm fibre in which to pupate. A fortnight later the metamorphosis is complete and yet more long-snouted adult weevils are gung-ho to reproduce on the same tree, or fly off to others within a radius of five kilometres.

A single palm tree may play host to all stages of this life-cycle at any one time. Four successive generations of weevil may be produced in a single tree in a single year.

Meanwhile over in the pine woods, female Pine Processionary Moths lay their eggs in summer, covering and fastening them to the needle foliage with scales produced in their abdomens. About a month later, the caterpillars emerge and go a'wandering about the branches, spinning temporary shelters here and there before settling down into communal nests.

Each nest consists of a silky ball - larger than a tennis ball, smaller than a football - secured in a sunny spot in the outer foliage. The nocturnal caterpillars occupy it throughout the autumn and early winter, snuggling up together during the day, venturing out to feed in fine weather at dusk, returning at dawn. Ah, what a life!

In February and March the fully-grown caterpillars descend and trek overland in long, head-to-tail lines to find suitable soil in which to bury themselves, form cocoons and pupate so that the whole life-cycle can start again.

The processions that give this insect its name are fascinating. Each individual has a gland at the tip of the abdomen that serves as an attachment for the next-in-line. Columns a couple of metres long are common. Twelve-metre columns formed by as many as 300 individuals have been recorded.

The lead caterpillar is not a leader as such. It gets the number one slot by chance and if for any reason it becomes detached from its followers, the second-in-line takes over without much fuss. If the line-ahead formation is broken further back, however, the caterpillars immediately come to a standstill and remain flummoxed until they manage to restore unity or proceed as two separate columns.

Should you encounter a procession, by all means watch it – but also watch out! Be very careful that your natural curiosity keeps its distance. If disturbed, the caterpillars release fine, toxic hairs that float free and can cause severe skin rashes and eye irritations in humans. This defence mechanism is extremely effective against domestic animals too, especially nosy dogs.

The processionary caterpillars have a few other worries. Natural predators include the Hoopoe whose long curved beak is an ideal implement for hoiking pupae out of the ground. Some species of ant prey fiercely upon the young caterpillars. And those toxic hairs don't seem to bother the Azure-winged Magpie, the subject of my next Nature Watch article, next weekend.

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