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Insects can be garden pests, but some are actually beneficial

Editor's note: This is the latest in a series of biweekly columns from the Becker County Master Gardeners, who are part of the University of Minnesota Extension.

Insects in your garden, such as the aphids found on this kale leaf, can be both helpful and harmful. Integrated Pest Management, or IPM, can help you determine the difference, as well as how to mitigate damage through non-chemical means. Chemical pest removal should only be used as a last resort. (JoAnn Dobis / Special to the Tribune)
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Of the 1.8 million species of animals on the planet, 1.2 million of them are insects. A gardener may think every one of them is in his or her garden on any given summer day, though not all of them are there to cause problems.

Insects have a terrible reputation, and perhaps rightly so, because they kill more humans than any other animal. Thankfully, most insects we see in the garden wouldn’t be considered pests, though some can seem quite destructive, not to mention ugly, when they show up on our flowers and vegetables.
Many of the insects we see are beneficial, however. They can be pollinators, predators of insects that are truly pests, parasitoids, or insects simply living in harmony with the beautiful plants in our gardens.

Some of the most common questions a Master Gardener may get from hobby gardeners are about insects. Advice is sought about what to do when they are present in the garden. The strategy for dealing with insects has changed over the last few decades. The approaches we previously took may have contributed to decreasing populations of pollinators. This may have contributed to the difficulty of keeping bee colonies healthy, the overgrowth of insects tolerant to chemicals we use to kill them, and an imbalance of nature’s methods to maintain control of “pests."

Today we encourage gardeners to learn something called Integrated Pest Management, or IPM. The most important part of IPM is our frame of mind. We no longer want to eliminate most insects. We want to accept some insect damage as part of nature’s grand scheme — to manage the garden situation for semi-peaceful co-existence, while accepting tolerable damage. Resorting to chemical pesticides is a last resort.

IPM is defined as a sustainable pest management strategy that focuses on long term suppression of pest problems through a combination of cultural, physical, biological, and chemical tools that minimizes economic, health, and environmental risks. Here's how it works.


  • Step one of IPM involves cultural processes. This is how we design and take care of our gardens. Avoiding a long row or group of one type of plant discourages insects that favor that one plant over others. Wood mulch between plants encourages the presence of beneficial insects. Removing weeds deprives hungry insects of places to hide. Removal of dead or diseased plants is also helpful. One could consider “trap plants,” which is a plant sacrifice, of sorts, to divert insects from the valuable plants.
  • Step two of IPM is the use of mechanical measures. This would include physically removing insects from the plants with a spray of water from the hose — as one would rid Rudbeckia of aphids — or by picking them off by hand, as one would remove potato beetles. The use of diatomaceous earth is also considered a mechanical measure. This soil is so fine that it abrades the insect’s cuticle, resulting in its death from drying out or by plugging up its breathing pores, so it suffocates. This works well on slugs by wearing a hole in their undersides. The use of sticky traps is a mechanical trick as well. These should NOT be used with attractants. Netting can also be used to keep insects from your plants.
  • Step three is referred to as biological control. Parasites can be encouraged in one’s garden area. We all know lady bugs and lace wings eat aphids: We do not encourage buying them and letting them loose, but things can be done to encourage their presence. Many wasps are predators of insects. Microscopic worms, known as nematodes, can be used to control grubs and their adult stage Japanese Beetles. Bacterial species such as Bacillus Thuringiensis control leaf-eating caterpillars.
  • Step four is chemicals. Regarding chemical use, one has to think deeply about the philosophy of growing plants and decide if the damage we see is tolerable, and if killing the insects that are causing damage is going to make a difference. We also need to realize that the use of chemical insecticides has far-reaching effects. Is that caterpillar in your broccoli going to kill you? Nope, it’ll come to the surface if you soak the vegetables in water. Tired of seeing those maggots in your raspberries? Sure, but if you let them have it with some nicotinoid insecticide, it will kill them — but it will also kill the pollinators and maybe the birds that eat the insects, or other insects that eat the pesty insects. If you decide to use chemicals, make sure you know what the pest is and make sure the product is labeled for your pest and your problem. Always follow the directions.

When insect damage is spotted in your flowers or on your vegetables consider Integrated Pest Management principles and try to focus on the long-term effects of what is done to control this damage. Our level of tolerance may need to be adjusted. Our knowledge of insects may also need to be increased so we can alter the environment in non-chemical ways to discourage their presence. Specific questions about insects and IPM can be answered by contacting any of the Becker County Master Gardeners, or by searching the University of Minnesota Extension websites.

The Becker County Master Gardener program is operated by the University of Minnesota Extension.

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