Systemic insect pest control for trees

Since the soil has now been thoroughly soaked by rainfall, it's a good time to apply the systemic insecticide Merit, where it has been determined to be necessary. Its use has become so widespread, I decided that it would be timely now to give some background on it.

Around December 1997 I first started to hear about a new systemic insecticide that sounded very promising, according to the literature and word-of-mouth in the trade. As a systemic it could be used to quickly and drastically reduce difficult to control insect pest populations in tall trees via the roots, without a heavy-duty spray rig and without spewing large amounts of pesticide into the air. In addition, it was of a different class than the smelly systemic poisons I had previously encountered on garden center shelves.

Having read Rachel Carson's "Silent Spring," being a longtime fan of organic gardening, and working for a company that avoided the unnecessary use of pesticides, I viewed this wonderful new product, with a healthy skepticism -- not outright disbelief; just suspended judgment. I could not help but think of DDT, another insecticide once viewed as a wonderful new product that had become an environmental nightmare.

I soon came to find out that the active ingredient, imidacloprid, is also found in Advantage, a flea treatment for pets, which, on the family vet's advice we had used with excellent results, breaking the flea life-cycle for our dogs.

Discovered in 1985, and introduced into the marketplace in 1991 by Bayer, imidacloprid is in a class of chemicals called neonicotinoids. The "nicotin" part of the name refers to the related, naturally occurring chemical, nicotine.

Some of you gardeners might recall that nicotine was the active ingredient in a houseplant insecticide called Black Leaf 40, which is now banned in the United States. Concentrated nicotine alkaloid and sulfate are apparently too potent to be considered safe. The dermal LD50 for rabbits in testing was 50mg/Kg. In other words one-twentieth of a gram applied to the skin could kill a 2 pound rabbit!

Pretty scary isn't it? But that's for an unnatural concentration of a naturally occurring chemical that breaks down rapidly in the environment. Though not the same as nicotine, the synthetic imidacloprid is similar in some ways. In very low concentrations it acts very quickly, interfering with acetylcholine on nerve receptor sites. Quickly degraded by ultraviolet light, it breaks down in the environment rather than moving up the food chain like the infamous DDT.

In terms of toxicity to mammals, imidaclopid is much less concentrated. The LD50 (rats) is greater than 2000 mg/Kg, that is about 40 times "weaker" than nicotine.

Checking around the local market place I found imidacloprid still available as Advantage for flea control, through a vet's office, with no reports of harm to pets.

In garden centers it is available in various formulations by Bayer as 'Advanced Garden' products, at very low concentrations, with labeling for numerous pests, including "bronze birch borer."

For agricultural use, imidacloprid is available from Bayer to growers as 'Provado', a foliar spray, and 'Admire' a liquid applied to the root zone. Labeling includes control of glassy-winged sharpshooters (GWSS) on grapes, so we could see the use of this material skyrocketing in the future, if a local infestation of GWSS should occur.

Label cautions say that imidacloprid is highly toxic to aquatic invertebrates, and bees directly exposed to direct treatment or residues. The Admire label says that it is "moderately toxic" to mammals, moderately to slightly toxic to birds, and practically non-toxic to freshwater fish.

At a seminar on tree pest and disease management, one of the speakers Dr. Tim Paine of UC Riverside, reporting on the work of the late Prof. Don Dahlston of UC Berkeley on eucalyptus pests and beneficial insects, stated that Merit applied to the roots of eucalyptus trees for the control of lerp psyllid turned up in the nectar of the flowers at concentrations of 550 parts per billion, about double the concentration that can kill the nectar-feeding adult beneficial insects (Syngaster lepidus) that control the psyllids.

There are some interesting factors to consider before reaching for the jug of pesticide.

Bill Pramuk is a registered consulting arborist with Britton Tree Services Inc. Please send questions to bpramuk@pacbell.net.