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How to Manage Pest Insects

Insects can range from beneficial to destroyers of your favorite plants.  Recently Carol Allen, one of our resident speakers, gave a talk where she discussed different types of insects and how to keep the peskier ones at bay.  She is a science-based horticulturist and so follows the dictates of scientific studies, not the wives’ tales passed down for generations.  The following is what I gleaned from the first part of the talk: how to keep your plants pest free in general, using eco-conscious practices.  Later, I will post the second part of the talk: identifying and targeting specific pest insects.

First, start with healthy plants.  A stressed plant is way more prone to insect damage than a healthy one.  This means making sure it gets the right amount of sun, water, air and nutrients.  Below relates to plants in the ground, not containers:

  • Have your soil tested. This way you can choose plants based on your soil’s pH (for example, if you have alkaline soil, maybe don’t plant an acid-lover, like blueberries); the composition (sand/silt/clay); nutrients (e.g. if your soil test comes back high in phosphorous, don’t add fertilizer with lots of phosphorus); and organic matter (you only want 5% by volume, so don’t add compost if you’ve achieved that).  You can get your soil tested by the University of Delaware (click here to learn more).
  • Make sure your plant is getting the correct amount of water. Don’t just spritz it every day after work, instead always check the soil to see if it feels dry and if it needs it, water thoroughly and deeply.
  • Soil should always be covered with mulch (Carol likes straw) or plants to prevent erosion.
  • Don’t fertilize. If you’ve seen to the above, the plant should be happy and shouldn’t need fertilizer.  It encourages plants to push out tender, succulent growth that insects love to eat and it can run off into the Bay, causing harmful algae blooms.  The exception to this is vegetables which are putting nutrients into the bits that you eat and so will probably need a little boost mid-way through the growing season.  Carol suggests using compost instead of fertilizer because it’s harder to over-do.

So, what if you have healthy plants that are being attacked?  Don’t use a broad chemical spray; it may be the easiest solution, but it will kill the good bugs too and that would be bad for your plants, not to mention the environment in general.  Below are some ways to control pest insects in a more targeted, environmentally friendly way.

Put up barriers.
If you’re growing something that doesn’t need to be pollinated, like cabbage, you can keep all insects out with low tunnels; that is, small hoops staked over your vegetables, covered with barrier fabric and secured to the ground.  If you’re just trying to keep out rabbits or groundhogs, you can make simple frames covered in chicken wire to go over your vegetables.

Mechanical removal.
There are a number of ways to do this.  Some bugs can just be pulled off and squished (you’ll have to get over the “ick-factor”).  Small, soft-bodied insects (like aphids) can be knocked off with a powerful jet of water (Carol calls this “power washing,” but you don’t need a pressure washer—just set the dial on your hose nozzle to a fan or jet spray).  This doesn’t kill the insects, but it breaks their mouth parts so they can’t eat anymore and will starve.  For beetles, you can knock them into a bowl of soapy water.  This has to be done early in the morning because beetles aren’t active in cooler temperature and so won’t fly/run away.  Tap a branch with beetles on it over the bowl and they will drop into it.  The soap breaks the surface tension of the water, so when the beetles land in it, they sink and drown.

Attract beneficial insects that like to eat pest insects.
If you see them, definitely don’t spray chemicals and think twice before power washing.  You can attract them by increasing the diversity of plants in your yard.  Here are some good ones to include: carrot, Queen Anne’s lace, dill, anise, fennel, cilantro, parsley, ageratum, sweet alyssum, blanket flower, cosmos, sunflower, marigold, aster, daisies, mums, black-eyed Susan, coneflower, coreopsis, mints, thyme, sage, oregano, bee balm, salvias, nasturtiums and poppies.  With the herbs, insects will be happier if you let them go to flower, though you might want to remove seeds from dill and fennel plants because otherwise they’ll come up all over.

Some beneficial insects include:

Lady Beetles: They just love to eat aphids. If you see them in your garden that’s good, but store-bought lady beetles may not be the best idea.  For one thing, they come from California where they hibernate in the hills and fly into the valley to feed, so it’s ingrained in them to fly away to find food once they warm up.  Even worse, they carry a predatory mite that they’re resistant to, but our native lady beetles are not.  So, not only are they not eating your aphids, they’re leading to a decline in native lady beetle populations.

Green lacewings: The adults are beautiful and harmless; the nymphs are voracious aphid eaters.

Parasitic Wasps: Many of these are quite small, and not a sting hazard.  There is one that will lay its eggs in live aphids.  The larva will then kill the aphids from the inside.  There is another one that will do something similar to tomato hornworms (though in my experience it might not kill them before they have a chance to take down your tomato plant).

Minute Pirate Bug: Mostly eat pollen, but will go on occasional aphid binges.

Tiger Beetle: Colors can vary from emerald green to brown.  They eat a wide range of pest insects, including caterpillars, flies and ants.  Fun fact: they run so fast that they blind themselves while in motion and have to periodically stop to relocate their target prey.  Their speed—5 miles per hour—is equivalent to a human being able to run 480 miles per hour.

Syrphid Flies: Looks like a hovering bee, but don’t worry, it won’t sting.  It’s another one whose larvae eat aphids.

Soldier Beetle: Larvae eat insects.

Yellow Jackets: Yes, they may instill fear, but they like to eat cabbage worms and other soft-bodied insects.  So if you find a nest in a more secluded part of your yard, cordon it off and leave them be.

Wheel Bugs and Assassin Bugs: Another in the “look but don’t touch” category.  They will both eat larger insects, but if you try to handle them, they will bite.

Using biorational chemicals.
These are pesticides that are more environmentally friendly and less likely to harm anything but your target pest.  (Click here for a pamphlet put out by Cornell University on insects and diseases that harm crops and organic ways to manage them.)

  • Horticultural Oil and Neem Oil: Both are good for general use and kill by suffocation (they coat the bug so air can’t get to it).  You shouldn’t use them when temperatures are above 90°F or humidity is greater than 80% because you could damage the plant and you shouldn’t use them on outdoor plants that are blooming because it could harm bees/keep them from getting to the pollen.  Carol prefers plain horticultural oil because it does the same thing as Neem and is cheaper.
  • Soaps: Insecticidal soaps are composed of potassium and ammonium salts of fatty acids.  They are effective against aphids and other small soft-bodied insects, but must come into direct contact.  Some household soaps may also work but can be phytotoxic (kill new growth).  Soaps may not be as effective as oils at eradicating an insect population because they don’t kill the egg stage.
  • Bt (Bacillus thuringiensis): Bacteria that kill the larval stage of insects.  There are different strains that target different insects, but all are safe to use around people and animals.  The spray is effective against caterpillars and mosquito dunks use a different strain to kill mosquito larvae.  Because it affects butterfly and moth caterpillars, you shouldn’t use it in your pollinator garden.
  • Spinosad (Saccharopolyspora spinosa): Another bacterium, spinosad affects the nervous system of insects, killing them in 1-2 days.  It has low mammalian toxicity and is considered a natural product.  A brand we carry containing it is Capt. Jack’s Dead Bug Brew.  It is effective against beetles, caterpillars, sawfly larvae and soft-bodied insects.  Once dried, it is less likely to harm beneficial insects because it must be ingested (meaning the insect must be chewing on the leaf).  However, if an insect touches it when before it has dried, it has the potential to kill indiscriminately.  Use caution and spray when when insects are less active (early morning or evening) to give it a chance to dry before they come in contact with it (especially when spraying flowering plants) and consider only using as a last resort.

Stephanie Fleming

Stephanie Fleming was raised at Behnke’s Nurseries in Beltsville. Her Mom, Sonja, was one of Albert & Rose Behnke’s four children. She was weeding from the moment she could walk and hiding as soon as she was old enough to run, so many weeds, so little time. Although she quickly learned how to pull out a perennial and get taken off of weed pulling duty.

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