Know Your Beneficial Insects

Credit: Elizabeth Cronin

Hover flies – also called syrphid flies – are among the most voracious and hardy of our native aphid predators. The adult flies are striped black and yellow or black and white and about 1 cm (1⁄2 in.) long. They are strong, fast fliers, with a distinctive habit of hovering before flower blossoms like tiny hummingbirds. The adults are vegetarians that eat only pollen and nectar.

It is their immature (larva) stage that feeds on aphids (see photo). Hover-fly larvae are tear-drop shaped, legless and rather ugly, but just one larva can eat over 400 aphids. They are a mottled-tan or light-green colour, so you have to look closely to see them among the aphids. You can increase the numbers of larvae attacking aphids in your garden by planting flowers that attract the adults. Once the female flies have filled up on pollen and nectar, they search out nearby aphid colonies to lay their eggs. Plants that particularly attract hover flies are: feverfew, dill, coriander/cilantro, sweet alyssum, yarrow, lavender. – Linda Gilkeson, Ph.D., author of West Coast Gardening: Natural Insect, Weed & Disease Control