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Miners Are Easy To Kill!
by John Harmon
July 13, 2003

I've had a few people ask me how the robin is doing that took over my tractor as a preferred nesting site. I'm happy to be able to report that three chicks hatched and they are growing rapidly. They have their eyes open and both parents are busy keeping their young bellies full. The adults are sure cutting down the bug population around here just to keep up.

Speaking of bugs I've also had a number of inquirers this year about the outbreak of attacks on all sorts of trees around town. This time it isn't vandals, it's a tiny bug called a leaf miner. Scientifically it's known as Argyresthia thuiella. This pest is a tiny caterpillar (Lepidoptera) that primarily feeds on the leaves of deciduous trees. This year they seem to be everywhere on all sorts of deciduous trees and bushes.

The adult is a tiny silver to gray colored moth with a wingspan of only one third of an inch. The wings have brown and black markings. The legs and abdomen are light brown. The moths can be found flying around in the early spring after eggs that have wintered over hatch. The female moths deposit pinkish scale-like eggs on the tips of plant foliage over a five-week period. Eggs hatch in two to three weeks into very small (one eighth inch-long) yellow-green hairy caterpillars or larvae with a reddish tinge and a shiny black head.

The larvae burrow into the leaf scales and eat the inside of the leaf. Feeding injury starts in the summer and reaches a peak in the fall. Larvae normally tunnel into the growing points, killing affected tips. They start mining from leaf tips and move down toward the base. Sort of like placer miners working their way up the creek looking for the mother lode.

Foliage damaged from a leaf miner feeding is very easy to distinguish from healthy foliage. You can easily see the trails left inside the leaf. Holding damaged leaves up to the light you can even see the larvae inside the leaf tissue if you have sharp vision.

Most trees will generally survive the attack if they are otherwise healthy. Continued feeding by heavy leaf miner populations may kill some twigs and branches and it certainly doesn't look good but the tree should survive. The problem is that the larvae over winter within the mined leaves and feed for a short period of time in the spring before entering a pupa stage. This stage lasts approximately three to five weeks. Adults chew an exit hole from the mined leaves in late spring to early summer. There is one generation per year in the north.

There are some things you can do. Pruning out heavily infested twigs or branches and placing debris in a sealed bag or other container removes the leaf miner larvae. It will not only stop them from doing any more damage to the tree but prevents them from becoming adults and laying more eggs. The leaf miner is highly susceptible to natural enemies like parasitic wasps because they cannot escape being located within the leaf. The natural parasite may not be able to control a large outbreak like we are seeing this year.

Besides the environmentally friendly method of removing leaves containing the pest and disposing of them there are also some pesticides that are capable of penetrating the leaf surface and killing the larvae. They include abamectin (Avid), acephate (Orthene), and chlorpyrifos (Dursban). These are fairly potent pesticides and should be used with caution. If they can penetrate the surface of a leaf they can probably make it past your skin so follow the directions!

If you have the time I recommend picking off any affected leaves by hand and tossing them in the wood stove. Your morning fire will not only warm your cabin but it will warm your heart to know that you are cutting down the leaf miner population. If you get them before they hatch from the leaf you will have fewer to deal with next year.

With both the birds and bugs hatching you can tell that it's going to be a busy summer. Killing the leaf miners is probably much easier than getting rid of the Yukon placer miners. Just ask the feds! Placer miners tend to be much tougher, resistant to even the nastiest, ill-conceived legislation!
 



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