To Spray or Not to Spray

Part III The Gypsy Moth

by M.J. Harvey November  2004

Back in the days when I was semi-gainfully employed I would attend the summer meetings of the AIBS (American Institute of Biological Sciences), which are held in a different place each year. The many component societies hold simultaneous lecture series, poster presentations and field trips.

One year, in Pennsylvania, a session was devoted to the biology of the gypsy moth. On a field trip we went to some unremarkable scrubland near State College where the leader explained that in the previous year a population of gypsy moths had reached on of its periodic peaks and then collapsed. What had happened was that over a number of years the local gypsy moth population had gradually increased in size until, in the previous year, there were so many caterpillars that they ate off all the leaves on the deciduous trees, descended to the ground, ate off the herbs and grasses (not their favourite food) and then died of starvation before they reached maturity. The region looked like a moonscape that previous year we were told, but at the time of the trip had no visible infestation. What would happen next is that the very few moths remaining would form a scarcely detectable population that would gradually increase over a few years until another peak.

Here in Victoria we have a native insect, the western tent caterpillar, which goes through a cycle of roughly eleven years and is coming to one of its peaks in a year or so. The difference between it and the gypsy moth is that it has evolved in the west in parallel with a suite of parasites ranging from viruses and bacteria to various tiny ichneumon wasps. These parasites increase their populations (but with a lag) along with the tent caterpillar and eventually control their numbers.

The European gypsy moth was deliberately brought to the eastern United States over a century ago in the mistaken belief that it would form the basis for a lucrative silk industry but this turned out to be illusory and a few escaped, establishing populations which have been spreading in the east ever since, reaching Canada some years ago. It is now a major pest.

One of the reasons that the gypsy moth has been able to establish itself so effectively is that it has outrun its controlling suite of parasites. The person bringing the first specimens across the Atlantic didn’t bring any of the parasites and likely didn’t know of their existence, and no one has been able to introduce them since. The caterpillars are very hairy and birds don’t eat them because they taste bad – although I can’t vouch for that.

Here in British Columbia we have a thriving port at Vancouver with ships from across the Pacific bringing in containers of goods from Asia. Many of the goods are landed on pallets that are made in the country of origin as cheaply as possible from unseasoned local woods. Scientists at the Pacific Forestry Institute have found that a surprisingly large number of insects will hatch out from these pallets over a period of months. Among the insects hitching a lift across the Pacific either on pallets or the ship’s superstructure is the Asiatic gypsy moth.

The Asiatic gypsy moth differs from its closely related European cousin in small details beloved of entomologists. From a gardener’s point of view they have the same voracious appetite and, believe it or not, the parasites that control Asiatic’s population size are stuck way back in Asia waiting for immigration papers.

A large number of gardeners, orchardists, foresters, scientists and politicians, the latter fearing US embargoes, want to prevent the gypsy moth from getting established in B.C. You do this by jumping on the small initial colonies as soon as they occur. And how do you find new infestations? You use gypsy moth pheromone, which was, I seem to remember, the first pheromone to be analysed and then synthesized.

In terms of their reproductive biology the female moth just sits on a twig and releases her pheromone into the wind and the male moth detects it with his large antennae. The male moths are incredibly sensitive to traces of this perfume – estimated at as little as a few hundred molecules – and can detect a female kilometres upwind. This led to the detection method that consists of traps containing a trace of the synthetic hormone. You may have noticed these traps hung in bushes, they are triangular in cross-section and brightly coloured. The traps attract only the males, but having found males, that tells you there is a gypsy moth population in the vicinity.

Having detected a population, measures can be taken to eliminate it but this is the point at which objections and the unspoken fears of citizens come to the fore. There is, in fact, a big controversy over how, or indeed whether, introduced gypsy moths should be controlled. It has got very political with citizen groups, petitions, and lobbies, protesting against spraying. The objections are many and it is the point of this article to discuss and analyse them.

Let’s take the do-nothing proposition first. This attitude comes from people who don’t want to be sprayed with anything, by anyone. The feeling is that it just means a few caterpillars on your plants and heaven knows there are always some of those around so why bother about a few more – what is the fuss about? Well the fuss in this case is about population density. The favourite food of the larvae (the adult moths don’t eat) is the leaves of deciduous trees, especially fruit trees, alders, maples, oaks, birches, etc. Rhododendron growers can breathe a sigh of relief, they are not touched except for deciduous azaleas. But deciduous trees at peak moth populations are stripped. For instance the cherry trees in the streets of Victoria will appear to be in their winter state by June.

Given no control measures the Law of Unintended Consequences will spring into action and the non-organic gardeners are going to get very indignant and will shower their own properties with all and any chemicals they can get their hands on. So by opposing the use of spray, there will be more spray used including some possibly unwelcome ones.

So we have to consider aerial spraying with Bt, the bacterium that kills the larvae when they eat leaves with the bacterium on the surface. When Bt was introduced I thought what a beautiful solution this was to the problems that previous generation of pesticides had engendered. Such an organic solution – using a naturally occurring soil bacterium that only attacks a limited range of insects. Imagine my surprise at the vocal, even vehement opposition to Bt that has cropped up at public hearings on the subject over the past few years. Let me go into some of the objections:

Objections to Bt Spray

1. Spraying from planes is too indiscriminate. Why not use hand sprays from the ground? This appears to be the fear of Big Government spraying the helpless citizens. Poison gas warfare comes to mind. The answer to this is that spraying from the ground will not get to the tops of full alder and maples in dense forest.

2. Bt might infect our children. This is a very emotive subject. Children, with immune systems not yet fully developed, might be susceptible to Bt. It is after all a bacterium and, as years of TV have drilled into our minds, bacteria are dangerous, evil and have to be eliminated by using such-and-such a mouthwash, toilet cleaner, tile spray of disinfectant soap (does anyone remember Lifebuoy?). The conditional tense ‘might’ tends to be used a lot in presentations. I have never heard of a human infection with Bt and given the differences in structure of the cells lining a caterpillar’s gut and those lining a human digestive track one would not expect one.

3. People with compromised immune systems – transplants or HIV – might become infected. Highly unlikely but such people are in much more danger from the common human infectious diseases, not an insect pathogen.

4. The spray will kill off the local beneficial insects such as butterflies and bees. The way Bt works is that it has to be consumed on a leaf shortly after the trees are sprayed. Thus LGC’s (little green caterpillars – the winter moth larvae) are eating young tree leaves at the same time as the gypsy moth and are affected, but the vast majority of other moths and butterflies are timed at a different season and are not affected. Bees are not leaf eaters and hence not susceptible. In the years when areas were sprayed there was no shortage of butterflies.

5. Bt Spray is not organic – it is a chemical spray. Yes, good point. In order for Bt to be effective a couple of things have to be added. One problem with bacteria is that on the surface of a leaf the UV of the sun will kill them, so a sunscreen has to be added. The other problem is that rain will wash them off so a ‘sticker-spreader’ is added to the mix; this is a detergent/thickening agent. These additives are not toxic or out of the ordinary – they are the sort of thing many people voluntarily put on their skin, but to the organic lobby they transform Bt spray into a ‘chemical’ and hence completely unacceptable.

These arguments will be repeated in public hearings near you should more gypsy moth populations be found. This may be as soon as next spring. I shall support spraying with Bt. But based on past experience there will be many citizens very fearful of what the government is doing to them.

I shall try to finish this series in a future newsletter. When I was a young student of chemistry I thought I knew what the words ‘chemical’ and ‘organic’ meant. But the world changes. There are now extensions of usage some of which reverse my youthful certainty. So I shall discuss the organic movement.

To be continued.