Gardening is a significant outdoor activity of many urban and suburban homeowners. In fact, over 67% of American households have gardens. For many urban and suburban residents, lawns and gardens may represent the closest green space available to them. But urban gardens contain more than green plants, flowers, and vegetables: home lawns and gardens also have numerous insect pests, weeds, and plant diseases, many of the same kinds that affect farmers throughout the state.
To combat these pests, most urban and suburban homeowners respond by applying pesticides. Although homeowners may feel the use of pesticides in their own yard and garden is only of minor significance, collectively homeowners spend over $11 billion per year on pesticides. Pesticide use on home lawns and gardens, on a per-acre basis, actually exceeds the use of pesticides in agricultural crops. Gardeners made aware of these facts want to replace use of chemicals in their yards and gardens with biologically based tactics. Knowing about alternatives could reduce household pesticide use dramatically, but the average gardener is unaware of alternatives or how to use them. Unfortunately, many alternative approaches have taken the form of "snake oil," being either anecdotal or untested solutions.
A garden thriving without the use of pesticides but with biologically based
strategies for pest control.
We tackled this problem head-on. In a joint project between entomologists at the Illinois Natural History Survey and Purdue University, we developed a training and research program for Illinois and Indiana gardeners, teaching about alternatives to pesticides and biological control tactics to use in home yards and gardens. To date, we have taught over 250 Master Gardeners about pests, natural enemies (predators, parasites, and diseases of pests), and biological control tactics that can be used in their gardens. During workshops held in cities in both states, we surveyed gardeners to learn about their current pest management practices, use of pesticides, and awareness of alternatives. Gardeners were then re-surveyed after one year to see if they altered their practices as a result of the training.
We found that training gardeners about biological control and alternatives to
pesticides greatly affected their use of pesticides and alternative tactics.
Before training, 63% of all gardeners relied mostly (using them either always
or usually) on using insecticides against insect pests. One season later, only
28% of gardeners relied mostly on insecticides (Fig. 1).
Figure 1. Percentage of gardeners using insecticides more than once per season
before and after training in biological control.
Many gardeners totally quit using pesticides--42% of gardeners used no conventional insecticides after training, versus only 15% of gardeners who used no conventional insecticides before the training workshops. Others increased their use of alternative tactics, such as companion plantings and mulches.
Reducing pesticide use in the home garden is one benefit of the program. Still another is developing alternatives for gardeners' pest problems. We taught gardeners about conducting research and helped them to become "volunteer researchers," testing potential biological control tactics, using their ideas and their gardens as test plots. We wanted the experiments conducted scientifically to see if the tactics worked and we could make recommendations to other gardeners. Gardeners conducted four research projects with our guidance and support: releasing Trichogramma wasps weekly against larvae of cabbage butterflies; comparing the numbers and kinds of predators caught in pitfall traps situated in mulched and unmulched potato plots; testing to see if spraying sugar water onto tomato plants attracted or retained predatory ladybird beetles; and spraying beneficial nematodes onto iris plants to combat iris borer.
Two of the volunteer research projects had enough participants to yield useful results. Gardeners who released tiny (< 1/64-inch-long) Trichogramma wasps weekly reduced levels of cabbage worms greater than fivefold in 1998: from an average of 0.59 caterpillar larvae per cabbage plant where no wasps were released to 0.11 larvae per plant with wasp releases (Fig. 2).
Figure 2. Numbers of cabbageworm larvae per plant, with and without releases of Trichogramma wasps, in 1998 and 1999.
However, in 1999 no similar pattern was seen because numbers of cabbage worms were below pest status. Control plots had 0.12 larvae per plant, versus 0.10 larvae per plant with releases. Thus, although releasing wasps could reduce the cabbage worm numbers from pest to nonpest levels, the wasps could not reduce numbers of cabbage worms when they were already low. Further, weekly releases of wasps cost more than gardeners were likely to pay (about $50 per season). Further tests will be conducted to see if we can reduce the number of releases, start later in the season, or quit sooner to make the tactic less expensive. Gardeners also found that spraying their iris beds with nematodes in early May reduced the damage from an average of 24% of rhizomes with iris borer damage to 34% of rhizomes with damage in control plots. Spraying nematodes later in the summer, when soil temperatures were higher, had a greater effect. Where nematodes were applied in June, only 11% of rhizomes had evidence of boring by iris borer compared to 24% damaged rhizomes in control plots. This tactic will be pursued again in 2000 to see if the results hold true. The pitfall trap and sugar water experiments did not show any differences between treated and control plots, though small numbers of plots may have affected the results.
The real test of the project's success will be the expansion over the next few years. We showed that gardeners trained in biological control adopted practices to reduce their use of insecticides. Simple education about biologically friendly alternatives greatly changed the gardeners' behavior. Greater adoption of biological control and non-chemical pest management alternatives will help keep urban gardens green--and friendly to the gardeners.
Robert N. Wiedenmann, Center for Economic Entomology; Clifford S. Sadof and Robert O'Neil, Purdue University
Charlie Warwick, editor