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MOF&G Cover Fall 2011

 


  You are here:  PublicationsMaine Organic Farmer & GardenerFall 2011English Editorial   
 Polycultures as Pest Control Opportunities Minimize

Cedar waxwing nestlings
Cedar waxwing nestlings in an organic Christmas tree. Diazinon, an insecticide recently approved in Maine for use on non-organic Christmas trees, can be toxic to birds, bees, fish and other wildlife. English photo.

By Jean English
Editor, The Maine Organic Farmer & Gardener

(Disclosure: The author grows organic Christmas trees.)

The balsam gall midge is a fragile creature. According to Ronald Kelly*, a forest insect and disease specialist with the state of Vermont, the orange-colored adults, just over 1/8 inch long, emerge from the soil in spring, when females lay eggs on the needles of newly opened balsam fir buds – unless conditions are so windy that the midges can’t land there. If they do lay eggs, then two to three days later the eggs hatch into larvae, and needle tissue forms a gall around each larva. The larvae exit these galls in the fall and drop to the soil to overwinter.

That’s when trouble can begin for conventional Christmas tree growers, as needles that supported midges turn brown and they, too, fall from the tree. The top half of affected trees can be defoliated, making the tree unmarketable – for a while. Trees can be sheared and will recover and be salable within a couple of years, but most growers are reluctant to wait longer to sell a crop that has already taken eight or so years to reach marketable size.

Midge populations are cyclical, peaking every six to eight years. When populations peak, numerous predators and parasites, including various wasp species, such as a tiny black wasp in the genus Platygaster, subsequently peak and can decimate the midges.

Even more important is another midge – “the good midge” – that takes up residence in galls made by the balsam gall midge and seems to kill “the bad midge” larvae by preventing them from feeding.

Kelly notes that some growers who sprayed the insecticide chlorpyrifos two or more times per year consistently had problems with the balsam gall midge, probably because the insecticide killed the good midge and so interrupted the cyclical population dynamics of predators and parasites.

This past June, the Maine Board of Pesticides Control approved a Special Local Need registration request for conventional Christmas tree growers to use the organophosphate insecticide diazinon to kill balsam gall midges. According to its label, diazinon is highly toxic to birds, fish and other wildlife, and to bees exposed to direct treatment or to residues on blooming crops or weeds. That’s a high potential price to pay to kill a delicate little fly that is readily controlled by time and by nature’s cycles.

Interestingly, balsam firs growing in natural forest settings don’t succumb to midges the way those grown in open monocultures do. Shaded trees are less susceptible to midges, and natural enemies of midges keep populations in check in forests.

But most Christmas trees are grown in the open, in monocultures, with herbicides used to defoliate a strip of ground under the trees and with vegetation between tree rows kept short. Parasites and predators hardly have a chance.

If Christmas tree plantations were part of an organic polyculture and if natural enemies of the midges were allowed to proliferate, overall income could probably equal or exceed that of monocultures – and other organisms that enjoy Christmas trees (including humans!) would be in less peril.

Polycultures might include cows grazing in tree plantations, pastured poultry rotated through plantations (perhaps giving some control by scratching under trees and eating midge pupae and adults), or economic tree and shrub crops interplanted with Christmas trees.

Flowers that attract beneficial insects could be grown, with some cut for bouquets to sell – perhaps even cut-your-own bouquets that would bring Christmas tree customers to the farm more than once each year.

Tree crops – American chestnut for nuts, poplar for lumber, white oak for birds – might be intercropped here and there, just enough to diversify the planting.

The Rachel Carson Council (at www.rachelcarsoncouncil.org/index.php?page=birds) notes that the breakdown products of diazinon are even more toxic than the parent chemical; and the summer 2000 Journal of Pesticide Reform (at
www.pesticide.org/get-the-facts/pesticide-factsheets/factsheets/diazinon) discusses the toxicity of diazinon to birds. For example, robin eggs in Christmas tree plantations that were sprayed with diazinon hatched at a lower rate than those in unsprayed nests; a greater percentage of song sparrow nestlings died in Christmas tree plantations sprayed with diazinon versus those in unsprayed nests; bobwhite quail ingesting 35 ppm diazinon with their food produced fewer eggs than those fed uncontaminated food. And on and on.

And how many midges might birds consume, if the birds themselves aren’t consumed by diazinon? That would be far more interesting to study than the effects of diazinon on a sometimes somewhat problematic midge.

A Christmas tree grower might even use some of the space between tree rows to raise potted woody plants that feed birds, and sell those plants to the public. He might place beautiful birdhouses throughout the plantation, and sell birdhouses as well.

When growers stress ecology over chemistry, when they accentuate the positive rather than go to war with the negative, they create for themselves a wealth of possibilities for natural pest control and diversified crop growth. Without having to worry that particular crop combinations can’t be grown because a particular pesticide isn’t registered for each crop in the combination, leaving that pesticide out of the system enables creative and productive polycultures.

My own small Christmas tree plantation, which never experiences herbicides, insecticides, fungicides, miticides or other life-icides, supports thriving populations of swallows, mourning doves, cedar waxwings, robins, chickadees and other birds. Goldenrod, Queen Anne’s lace, valerian (another potential economic crop) and many other flowers grow between the trees and are alive with abundant populations of beneficial insects. Since the trees are just one source of our overall income, we can wait for them to recover from occasional pest damage before we sell them. But we have never lost even half a tree to the balsam gall midge.

Takao Furuno, in his book The Power of Duck, writes about switching from chemical-intensive industrial rice farming to a synthetic-chemical-free polyculture, in which ducks eat insects and snails that can harm rice plants, and ducks and fish eat much of the aquatic fern Azolla, a “weed” that was previously killed with herbicides. The remaining Azolla fixes nitrogen, and that nitrogen, combined with droppings from the ducks and fish, fertilizes the rice. Furuno sells not just rice, but also duck meat, duck eggs, fish meat and fruit (from trees that can grow around his paddy because he no longer uses herbicides). And his income is greater than it was when he grew rice alone.

It’s not the 1950s anymore. The post-war chemical boom has had enough unintended consequences. It’s time – beyond time – to look to ecosystems to help grow our crops.

* Management of the Balsam Gall Midge in Christmas Tree Plantations, by Ronald S. Kelly, 2009, at http://extension.unh.edu/resources/files/Resource000986_Rep2301.pdf.


  

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