Insect pests and the organic gardener

Posted Wednesday, May 23, 2012 in Features

Insect pests and the organic gardener

Japanese beetle, which can devastate many Maine crops and ornamental plants.

by New Maine Times Staff

Less troubling than disease for the organic gardener, but just as capable of causing great harm, are insect pests.

Insects and other arthropods are a natural part of the garden, and without them, we'd have no fruits and vegetables for the most part, because bees, wasps, flies, and even mosquitoes (the males, at least) help to pollinate our plants, while spiders help to control the pest population. But some arthropods are bad news, and bring nothing but devastation.  Also problematic are some below-soil worms and grubs that attack roots. 

Some well-known commercial pesticides and herbicides contain enzymes and salts (especially glyphosate) that are associated with Colony Collapse Disorder, a disorder that affects honey bee hives.  Since all farmers and gardeners ... whether they are organic or not ... need honey bees for pollinatiion,  the use of commercial pesticides and herbicides should be dramatically reduced.  Fortunately, there are nontoxic alternatives to the use of virtually all pesticides, and old-fashioned weeding in the typical garden (and weed prevention by mulching and garden fabric) goes a long way toward curtailing the need for herbicides.

The organic gardener's first line of defense is prevention. 

Companion planting for pest control

Consider the case of Japanese beetles.  This relatively recent imported pest loves roses, as many of us have learned to our horror.  But the beetle hates chrysanthemums, garlic, tansy, rue, catnip, chives, and a host of other plants.  Simply by planting companions with your roses, you may be able to deter Japanese beetle infestation.

There are plenty of other companion plantings that deter many other bugs.  This list comes courtesy of Gardens Ablaze.  (

Ants Catnip, Mint, Onion, Peppermint, Spearmint, Tansy, Wormwood
Aphids Catnip, Chives, Coriander, Dried & Crushed Chrysanthemum, Eucalyptus, Fennel, Feverfew (attracts aphids away from Roses), Garlic, Larkspur, Marigold, Mint, Mustard, Nasturtium, Onion, Oregano, Petunia, Sunflower
Asparagus Beetle Basil, Calendula, Nasturtium, Parsley, Petunia, Tansy, Tomato
Bean Beetle Rosemary, Savory
Black Flea Beetle Artemisia, Sage
Cabbage Looper Dill, Eucalyptus, Garlic, Hyssop, Peppermint, Nasturtium, Onion, Pennyroyal, Sage, Southernwood, Spearmint, Thyme, Wormwood
Cabbage Maggot Garlic, Marigold, Radish, Sage, Wormwood
Cabbage Moth Artemisia, Hyssop, Rosemary, Sage, Savory, Thyme
Cabbage Worm Celery, Thyme, Tomato 
Carrot Fly Leeks, Lettuce, Onion Rosemary, Sage, Tobacco, Wormwood
Colorado Potato Beetle Catnip, Coriander, Eucalyptus, Green Bean, Marigold, Nasturtium, Onion, Tansy
Corn Earworm Cosmos, Geranium, Marigold, Radish (gone to seed)
Cucumber Beetle Catnip, Marigold, Nasturtium, Radish, Rue, Tansy
Cutworm Spiny Amaranth
Flea Artemisia, Pennyroyal
Flea Beetle Catnip, Garlic, Mint, Rue, Sage, Southernwood, Tansy, Tobacco, Wormwood
Fly Basil, Pennyroyal, Rue, Tansy
Imported Cabbageworm Dill, Garlic, Geranium, Hyssop, Mint, Nasturtium, Onion, Pennyroyal, Sage, Southernwood, Tansy, Thyme
Japanese Beetle Ageratum, Arbortivae, Artemesia, Ash, Begonia, Boxwood, Caladium, Catnip, Chives, Cockscomb, Garlic, Hydrangea, Juniper, Pansy, Rue (with Roses & Raspberries), Tansy,  White Geranium, Yew
Leafhopper Dried & Crushed Chrysanthemum, Geranium, Petunia
Mexican Bean Beetle Marigold, Petunia, Rosemary, Savory
Mice Tansy, Wormwood
Mosquitoes Basil, Garlic, Geranium, Pennyroyal
Mole Castor Bean, Narcissus
Moths Lavender (combine with Southernwood), Wormwood, and Rosemary for an anti-moth sachet)E
Peach Borer Garlic
Nematode Calendula, Marigold
Onion Fly Garlic
Potato Bug Dead Nettle, Flax, Horseradish
Rabbit Garlic, Marigold, Onion
Snails Artemisia, Fennel, Garlic, Rosemary
Slugs Artemisia, Fennel, Garlic, Rosemary Sage
Spider Mite Coriander
Squash Bug Catnip, Mint, Nasturtium, Petunia, Radish Tansy
Squash Vine Borer Radish
Striped Cucumber Beetle   Tansy
Striped Pumpkin Beetle Nasturtium
Ticks Garlic
Tomato Hornworms Dill, Borage,  Calendula, Marigold, Petunia, Opal Basil
White Cabbage Moth Mint
Whitefly Basil, Marigold, Oregano, Peppermint, Thyme, Wormwood

Beneficial Insects and Arthropods

Gardens are a buggy world.  Every gardener knows that bees and other pollinators are needed, and that spiders eat bugs.  But many insects are predators of other insects, and the gardener can use this fact to her advantage. 

For instance, green lacewings are beautiful insects, snacking mostly on pollen and nectar, making them a valuable pollinator in their own right.  But in their larval stage, green lacewings are ferocious predators of some rather annoying garden pests ... the aphids.  A single juvenile lacewing (also known as the aphid-lion) can dispatch dozens of aphids at a single meal.  And they are always eating.

Ladybird beetles, usually called lady bugs, are also aphid killers.  But they also eat scale insects, thrips, mealybugs, and mites – all the pests gardeners despise.  And both the juveniles and the adults eat them. 

Praying mantis eat virtually everything, and they're not picky, which means that helpful insects can also find themselves dinner for the mantis.  But mantis can also handle large pests, such as caterpillars and hornworms, which the smaller lacewing larva and ladybugs cannot.

Minute pirate bugs also go after thrips, aphids, and mites.

Ground beetles - those bugs with the green metallic shine - have larva that kill many underground pests and pests right at soil level, including slugs, root maggots, cutworms, and, as an added bonus, the Japanese beetle grub.

Syrphid flies often wear bright markings of yellow-orange and black, and can be mistaken for bees. Like all flies, though, the syrphids have just two wings, so take a closer look if you see a new "bee" in your garden. Syrphid maggots crawl on garden foliage, searching for aphids to eat. They're quite good at squeezing in the curled up leaves where aphids hide, too. As an added bonus, the adults will pollinate your flowers. Syrphid flies are also called hover flies, because they tend to hover over flowers.

A gift from the sea - Diatomaceous Earth

Diatomaceous Earth (DE) is a powder made from the shells of diatoms ... single-celled algae that live in oceans, rivers, and the soil.  It works by slicing open the exoskeleton of insects.  However, it is entirely nontoxic, can be used around human habitations, is said to kill bedbugs, fleas, ants, and other household pests, is often added to pet and organic livestock food to kill internal worms, is used in chicken dusting boxes to kill mites, and can be used as a dusting for fleas, ticks, and lice on your pets.  In short, it's harmless ... unless you happen to have an exoskeleton.

To use DE in your garden, dust the plants and the soil around them using a duster specifically for DE, or simply repurpose an old sifter and dust plants liberally.  Do this on a sunny, dry day with dry DE.  A little moisture will decrease its effectiveness, so dust at a time when you aren't planning to water for a while. 

You can also use DE when you are storing grains and other foods, since it is safe for consumption.  Make sure you purchase food grade diatomaceous earth -- there is one for swimming pools that isn't meant to be eaten.  Food grade has a less than 1 percent crystalline silica content ... the pool stuff can be up to 70 percent.

DE is a strong defense against crawling pests (it doesn't affect bees or other flying pollinators) that you can use with a clear conscience.

Plant oils that function as insecticides

Despite our best efforts, there may be times when you need to use a chemical to kill stubborn pests.  However, as an organic gardener, you are limited as to the type of chemical you can try.  There are two plant oils on the market that are acceptable in the organic garden -- neem oil, and pyrethrum, made from the oils of chrystanthemums. 

Neem is a botanical pesticide derived from the neem tree, a native of India. This tree supplies at least two compounds, azadirachtin and salannin, that have insecticidal activity and other unknown compounds with fungicidal activity. The use of this compound is new in the United States, but neem has been used for more than 4,000 years for medicinal and pest control purposes in India and Africa. It is not highly toxic to mammals.


Pyrethrum is the most widely used botanical insecticide in the United States. The active ingredient, pyrethrin, is extracted from a chrysanthemum plant, grown primarily in Kenya, Rwanda, Tanzania and Ecuador.

Most insects are highly susceptible to pyrethrin at very low concentrations. The compound acts rapidly on insects, causing immediate knock down. Flying insects drop almost immediately after exposure. Fast knock down and insect death don't, however, always go hand in hand; many insects recover after the initial knockdown phase.

Pyrethrins are highly irritating to insects; as a result, they may be used as "flushing agents," causing insects to come out of hiding, a desirable circumstance when you need to identify an insect that is hiding in the turf grass such as grubs or sod web worm.

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