Nets, Sprays or Flowers? Protecting a School Garden

Caterpillars in the Kale

1The garden club at Harmon Middle School grows kale for the cafeteria salad bar and squash for a fall farm stand. Last summer, green caterpillars of the cabbage white butterfly chewed holes in most of the kale leaves. The club wants to stop the damage, but the garden does more than feed people. Bees visit the squash flowers and carry pollen between them; a female squash flower that is not pollinated usually withers without making a squash. Many kinds of beetles, spiders and tiny wasps also live among the plants. Some of those tiny wasps lay their eggs inside caterpillars, and the young wasps kill the caterpillar as they grow. Adult wasps of this kind feed on nectar from small flowers. Pollination and natural pest control are ecosystem services: benefits people get from healthy ecosystems.

2Club leader Odalys Vance found the results of a trial that the county extension office ran last summer in its demonstration garden. Staff used four beds about 30 m apart. Each bed had the same kale variety, the same soil and the same amount of water, and 10 squash plants grew beside each bed. Each bed got one method for the whole summer. Bed 1 had no change. In Bed 2, staff sprayed the whole bed, squash included, once a week with an insecticide that kills many kinds of insects. In Bed 3, fine insect netting covered the kale, and the squash was left uncovered. In Bed 4, staff had planted a strip of native flowering plants along the edge of the bed the spring before, so it was in full bloom.

3Staff measured five things (Table 1): the percent of kale leaves with holes in August, the total kale harvest, the average number of bee visits to squash flowers in 10-minute watches, the number of kinds of insects and spiders caught in a sweep-net survey, and the fruit set, which is the percent of female squash flowers that grew into squash.

4The club has three criteria. It wants at least 6 kg of kale from each bed, well-pollinated squash for the farm stand, and as many kinds of living things in the garden as it can support. Its constraint is money: it can spend at most $50 per bed each summer. Table 2 shows the cost and upkeep of each method.