IPM: Physical Barriers

apples_browns-tacoma_20091016_lah_4005Let’s say you want to grow apples in your Colorado garden—a perfectly reasonable option for this area. You’ve selected a variety that’s resistant to fireblight (I discussed disease-resistant varieties in May), and your tree is thriving. In fact, after several years, it’s finally beginning to bear fruit. You pick your first juice, red apple, take a big bite, and… oh no! Yup, you find half a worm. Ewwww.

codling-moth-wikiOf course you want edible apples, so what do you do? The first step is to identify the pest. In this case, a quick Google search tells you that apple “worms” are actually the larvae of the codling moth. And the standard advice for preventing wormy apples is to spray, starting two weeks after petal drop: “Because insecticide residues last 7 to 10 days and moths are continuously present throughout the summer, apply a spray every 7 to 10 days….”

While large-scale growers may have to resort to spraying that often, what about homeowners who want chemical-free fruit? There is an alternative to all those pesticide applications: bagging.

You can use standard lunch-sack size brown paper bags. Choose which fruit you want to keep, thinning the fruit in the process. (Any apples left outside the bags will likely attract codling moths, increasing the population.) The UC Davis provides the following instructions:

Bagging should be done about four to six weeks after bloom when the fruit is from 1/2 to 1 inch in diameter. Prepare No. 2 paper bags (the standard lunch bag size that measures 7 1/4 inches by 4 inches) by cutting a 2-inch slit in the bottom fold of each bag. Thin the fruit to one per cluster. Slip the thinned fruit through the 2-inch slit so that it forms a seal around the stem and staple the open end shut.

Bagging apples is an example of a very effective approach to pest control: creating physical barriers. A physical barrier has the benefit of being completely non-toxic, is often easy (but time consuming) to apply, and can frequently be reused. The netting we use to keep birds out of fruit trees is an example; so are fences to keep out deer, rabbits, and other herbivores.

reemay-abundantlifeseeds-comSpun polyester row covers (Reemay is one brand) make excellent barriers. For instance, grasshoppers are highly mobile. Controlling them with chemicals is largely ineffective because they simply take one bite and move on. However, swathing plants in polyester keeps the pests out. The row covers have the added benefit on wind protection, important in our area, plus they provide a few degrees of frost protection.

The holes in polyester row covers are so small, even flea beetles can’t get through them. The covers also stop leaf miners, cabbageworms, and most other insects, making them an important weapon in our pest-fighting arsenal.

It’s vital to apply the row covers before the adult insects lay their eggs on your plants, or you’ll be fencing the bugs in rather than out! Also, anchor the edges securely (usually by burying them) so no bugs can crawl inside. Plus, you don’t want the wind depositing the fabric in your neighbors’ yards.

These are just a few of the ways that physical barriers can be used to exclude pests from our plants. Often, we need to be creative and devise a barrier for a specific situation. I’d love to hear from you—what have you built to protect your plants? How did it work?

Codling Moth photo: USDA, Reemay photo: AbundantLifeSeeds.com

3 thoughts on “IPM: Physical Barriers”

  1. Floating row cover has been crucial for my lettuce. I put it down right after I plant the seeds and just water right through it. And then I also have good luck keeping the squirrels from eating my tomatoes by carefully encapsulating the fruit in those plastic strawberry containers while they are still green on the vine. I open them and pick the ripe fruit when they are ready.

    1. Paper bags… the plastic ones would concentrate the sunlight, and the plastic would trap moisture. I think you’d have a gooey mess.

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