Garden Good Guys: Natural Enemies in the Home Garden

The home garden is a complex landscape of vegetables, flowers, turf, woody ornamentals and other desired, and in some cases, undesired, plants. For insects, the home garden represents an ecosystem, where food, shelter and myriad species of insects, millipedes, slugs, spiders, etc. are found. To most of us, the home garden is seen as a relaxing place where we tailor the environment to our aesthetic and physical needs. Alas, the home garden is also the repository of nearly one-third of the pesticides produced in this country -- indeed acre by acre your cousin Vinny's tomato patch has more pesticides than farmer Brown's soybean field! Fortunately, and to the readers of MBCN, not surprisingly, the home garden is also home to our friends, the natural enemies. To take advantage of the work natural enemies do (kill pests), we must first know which ones we have and when they are present in your garden. Using natural enemies to control pests reduces your need to use pesticides ... and lets you take a bite from cousin Vinny's tomatoes, right off the vine!

Garden Natural Enemies. First, let's define the turf (no pun intended). In this article we'll report on a two-year study of natural enemies of home vegetables. The study was conducted in six "typical" gardens in Tippecanoe County, Indiana. By typical we mean a 40' x 40' vegetable garden that included tomatoes, squash, peas/beans, cabbage/broccoli, some flowers (marigolds were most common), and other veggies such as carrots, beets and potatoes. Gardens were located in more or less "suburban" environments. Each gardener was responsible for the care and maintenance of his or her own garden. Most mulched (with hay), and we asked that they not use insecticides without consulting us (only a couple of applications were made).

We used pitfall traps to sample natural enemies, and visited gardens once per week during the growing season (early May to late September) to record plant growth, collect traps and observe natural enemies on plants (not reported here). We placed one ounce of antifreeze in each jar to kill and preserve whatever creatures fell in. (Note: Do not use antifreeze if you have dogs. They love the taste, but alas their kidneys can not process the stuff. We used water in those gardens where Rover may be around.) Thus, what we report here are the natural enemies that were active on the soil surface. Many of these species do climb plants in search of prey (pests), and a surprising number of species we commonly associate with living on plants found themselves pickled in our traps.

All natural enemies were identified (to Family level). As it turns out, pitfall traps are great at catching predators such as spiders and beetles, not so good at capturing parasites, and down-right lousy at catching pathogens. So, for the rest of this article, we'll switch from saying "natural enemy" to "predator" to reflect this bias in sampling methods.

Results. The take-home message is: "There's a mess of predators in your garden." Over the two years of study we captured and identified 1229 predators. This number represented 34% of the 3607 insects and other arthropods we caught in our pitfall traps. To complete the breakdown, 34% (1231) were "omnivores" (things that eat about anything, like daddy- long-legs and ants), and 25% fell into the "other" category ("non-pest/non-predators", such as confused parasites that flew into the traps, and tiny non-descript flies, etc.). Only 7% (248) of the 3600+ bugs we caught could be called "pests" (mostly leaf beetles, planthoppers and weevils). So, if you had to guess, about one-third of the bugs in your garden are predators.

And what were the top predators found? The number one type of predator found were ground beetles (carabids); about 58% of all predators found (717/1229) were ground beetles. The second most common predator was also a beetle, lady beetles. About 18% of specimens (221/1229) were lady beetles. Spiders, particularly wolf spiders, linyphiids (sheet-web spinning spiders) and crab spiders, accounted for 16% of predators. Finally, our friends the fireflies (the new State Insect of Indiana!) accounted for 4% of predators collected. The remaining 10% of predators included a motley collection of damsel bugs, stink bugs, tiger beetles and other types of spiders.

Overall, we found nearly equal numbers of predators in 1994 (576 of the "Top 4" predators) and 1995 (560, "Top 4"). We see a general trend for the number of predators to increase over time (Figure 1) during the year. August was the top month for predators in 1994, whereas September was the month with most predators in 1995. Looking at trends for ground beetles, lady beetles and wolf spiders, we see that ground beetles showed a trend towards generally increasing in numbers over time (Figure 2), whereas lady beetles tended to peak in number during August (Figure 3), and wolf spiders peak earlier in July (Figure 4).

Overall relationships of predator numbers to temperature (1995 was warmer than 1994) and rainfall (1994 was wetter than 1995) were inconclusive. Almost all gardens had the same types and relatively the same number of predators, although the predators in your garden will very much depend on your management practices. So, just what are these predators doing in your garden?

As the name implies, ground beetles are commonly found scurrying on the ground, usually at night, where they eat many kinds of soft-bodied insect prey (e.g., caterpillars). They also climb plants to find prey, but because of their nocturnal habits, the average gardener does not notice this activity. During the day, these beetles are found under logs, stones, and leaf litter.

Another night-time predator is the wolf spider. It also is commonly found on the ground, where it feeds on numerous types of small insects. If you want to increase or maintain either ground beetles or wolf spiders in your garden, try mulching or placing a few flat pieces of wood to serve as habitat and a resting place for your predatory buddies during the day. And yes, wolf spiders bite, so handling them is not a great idea. However, you are about 1500 times their size, so they are not going to pick a fight with you!

The top day-time predators are the lady beetles. Lady beetles are familiar to most gardeners, at least in the adult stage. However, the larvae are also predaceous and both feed on pesky pests such as aphids, scales, mites and insect eggs. Lady beetles eat more than bugs, and particularly enjoy pollen and nectars from plants. So, to make your garden lady beetle friendly, plant some flowers, particularly those that shed lots of pollen ... unless you suffer from hay-fever!

Another day-time hunter is the crab spider. These spiders get their name from their characteristic way they hold their legs -- out to the side, like a crab. These spiders climb flowers and plants to find a good place to sit and wait for prey. Often these spiders are highly camouflaged, looking like the flower they inhabit. These spiders eat various small insects, and are quite capable of capturing flying insects that visit flowers (e.g., bees, but the spiders do not attack enough bees to disturb pollination in your garden). If you want more crab spiders, plant more flowers.

Finally, sheet-web weaving spiders (linyphiids) spin a large "sheet" that they use to capture prey. The sheet not only acts as a food-trap, but also protects the spider from other predators. These spiders eat small to medium-sized insects, mostly more-active species such as beetles and flies. These spiders are common in most gardens, and you can help them live in your garden by not disturbing their webs and diversify your plantings to give them a number of "homesites." (Note: A good time to see spider webs in your garden is early in the morning following a dew the dew adheres to the webs and makes them visible. Enjoy a scenic wonder in your own backyard!)

Ok, so what can you do to increase the number and diversity of predators in your garden? First ... put down the spray can! You most likely are applying too much, too often anyway, and you most likely kill more good guys than bad. Learn non-chemical ways to control pests and more about natural enemies (MBCN is an excellent source!). Experiment with natural enemies like you do plant varieties and fertilizing schedules. You can attract some natural enemies with flowers or apply a sugar solution to the plants to simulate nectar sources natural enemies "crave". Mulches help natural enemies by moderating temperatures and providing hiding sites and alternative habitats. By diversifying your garden you provide more opportunities for natural enemies. Although weeds have a bad reputation in most gardens, they do provide natural enemies a home in your garden, so a little re-evaluation of your herbicide use and hoeing practices would be helpful. Finally, contact your local Extension service and ask for advice. Many are quite knowledgeable about non-pesticide alternatives especially if they read MBCN! A little social action goes a long way to helping your new friends the natural enemies. Finally, make a commitment to make your garden a place where you have a cacophony of little voices. Spiders, lady beetles, ground beetles, predatory flies, parasitic wasps, etc., can provide you with free pest control. Save a few bucks, make your garden a "zoo", and enjoy cousin Vinny's tomatoes right off the vine!

- Bryan Schmeiser and Bob O'Neil, Purdue University

We want to thank our Hoosier gardeners M. Firestone, K. Atcheson, J. Yang, D. Smith, N. Kriebel, and A. Coblentz for letting us use their gardens for this study. Also thanks to Indiana Master Gardeners for assistance with this project. Besides Bryan Schmeiser, predators were identified by Vince Burkle and Cesar Teixiera.

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