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Growing Fruits Organically

By Bonnie Ennis, Colorado State University Cooperative Extension agent, horticulture

Concerned with upsetting the environmental balance of nature, and accidental harm to oneself during backyard synthetic insecticide applications, many backyard gardeners are turning to organic alternatives that target specific insect pests, display extremely short-lived residuals or introduce no chemicals at all into the environment.

Following are some organic insect control suggestions for fruit production in your backyard garden:

Wormy Apples

Getting the worm out of the apple is a difficult task. The worm is the larval, or eating-machine stage, of the codling moth. For some control, try the following in combination with each other:

  • Thin apples to prevent apples from touching. This cultural practice prevents larvae from using apples as leverage to bore into adjacent apples.

  • Band corrugated cardboard around the trunk of apple trees when the larvae begin crawling down trunks. This event generally occurs with the first generation of codling moths from mid to late June, then is repeated again by the second generation of codling moth larvae from mid to late July. Check the banded cardboard weekly for larvae and pupae which hide inside the cardboard. Destroy these inhabitants by crushing.

  • Mix molasses and water using a 1:10 ratio in a wide-mouth container. Suspend this homemade "hooch trap" from the apple tree. The trap will catch adult codling moths (and a mix of other moths from the neighborhood).

  • Consider sustained releases of large numbers of Trichogramma wasps on an experimental basis. Recent research suggests some codling moth control by these little wasps, barely visible to the naked eye. Trichogramma wasps parasitize the eggs of codling moths. These wasps are not aggressive and are available through specialty gardening catalogues.

  • Use of Bacillus thuringiensis to kill larvae and summer oils to kill eggs have shown very little control of codling moth larvae in apples.

Alternatively, commercial apple orchardists use a botanical insecticide called ryania to control codling moth larvae. As a botanical insecticide, ryania is toxic, yet short lived (several hours at the most). It must be applied very frequently, and it is expensive and difficult to obtain due to high demand by organic orchardists. As a source, try specialty mail order catalogues. If you don't receive these catalogues in the mail, consider visiting the Denver Botanic Gardens Library (requires admission fee or membership) to browse through its selection of gardening catalogues.

Peach Crown Borer

Peach crown borer is a destructive pest of the stone fruits - peaches, cherries, plums. The larval stage grows and tunnels under the trunk bark and sometimes lower branches. A first year crown borer infection won't kill a stone fruit tree. Uninfected trees do not need preventative treatments. Following accurate diagnosis of crown borer infestation, apply insect parasitic nematodes (eelworm) as a drench to lower trunk and adjacent soil in late August - early September.

Aphids on Fruit Trees

Whether or not little soft-bodied aphids will reduce fruit yields and plant health depends on the tree species and the interested aphid. For example, apple and cherry trees are commonly infested with aphids, but the types of aphids (green apple aphid and black cherry aphid, respectively), which like to feed on the leaves of these trees, rarely cause enough damage to warrant control.

Peach and apricot trees are more likely to be attacked by the green peach aphid, which causes premature yellowing and dropping of leaves. Leafdrop leads to reduced crop yields and poor tree vigor. The first control strategy of green peach aphid is to monitor populations to see if natural predators, such as lady beetles and their larvae, show up to eat the aphids.

If nature doesn't eat these pests, then apply a dormant oil in winter to achieve control the following year. This oil suffocates aphid eggs, recognized as shiny, black, small spots grouped in clusters on branches in winter. Dormant oil has been approved through the Colorado Organic Certification Act as an organic insecticide.

Scale Insects on Fruit Trees

Oystershell scales are little, hardbodied, slow moving insects that remove plant juices from trunks and branches of many tree fruits. Oystershell scales may not be recognized as insects because they have legs only during the crawler stage, which occurs for a short time in the last two weeks of May. Gentle scrubbing of oystershell scales with a plastic scrubby will give control with scrubbing done on an "as needed" basis.

Slugs on Small Fruits

Slugs like to eat holes in strawberry, currant and gooseberry fruits. These slimy animals thrive in moist, cool conditions. Removing soil surface mulches where they hide during the heat of day, thinning plants to encourage air movement between plants and banding the lower trunks of currant and gooseberry bushes with a copper barrier strip reduces impacts on these fruit crops.

If slugs reside in some parts of your yard, but have not yet invaded the strawberry bed, you can also band the perimeter of the bed with a copper barrier to keep out these creatures. Copper barriers impart an electric shock in slugs when the ions in copper react with the body mucus of the slugs.

Experimentally, try aluminum foil in place of the copper strips around the lower trunks of currants and gooseberries. No one knows if aluminum will repel slugs as well as copper, but the cost difference and easy availability of aluminum foil might make such an experiment worthwhile.

Miscellaneous Insect Pests

Several other fruit tree pests, such as San Jose Scale, some mites and leaf rollers can be controlled with dormant oil treatment, applied in winter.

The key to success in using organic pest control is a willingness to accept small pest populations and a certain amount of imperfect fruit (especially true with codling moth worms in apples). In some cases, insect pest populations will be controlled naturally by predatory mites, lady beetle larvae, and insect loving birds.

Photos: Judy Sedbrook

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Date last revised: 01/05/2010