Snapped: Good guy flies

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I bet you think a fly is a fly… but I’m here to tell you that you couldn’t be more wrong! The next time you spot a fly in the garden, don’t let its homely appearance fool you. Often a victim of mistaken identity, tachinid flies look much like their repulsive, germ-spreading, housefly cousins, but they are in fact remarkably beneficial insects. The 1,300-plus species of tachinid flies in North America are parasites extraordinaire, helping to control many common garden pests, including cabbage worms, potato beetles, cut worms, grasshoppers, Japanese beetles, squash bugs, Mexican bean and cucumber beetles, gypsy moth caterpillars, and many others.

Adult tachinid flies measure ⅓” to ¾” in length. Females insert live larvae directly into the host, or, more commonly, they lay eggs on the bodies of host insects. After the eggs hatch, the resulting larvae tunnel inside and excavate the host’s body, eventually killing it. The larval flies (called maggots, as is the case with many “true” flies) grow fat and sassy inside their host then pupate into adults.

tachinid caterpillar

A pest caterpillar with two white, oval tachinid fly eggs just behind its head.

While their larvae’s carnivorous nature may seem less than couth, finding an adult tachinid fly buzzing around your garden is undeniably a very good thing. You can tell a tachinid fly from its vile housefly relations not only by the dark, bristly hairs on its abdomen, but also by its presence in the garden. Houseflies don’t eat nectar but adult tachinid flies do. As the larvae are controlling pests, the adults are filling the role of an important pollinator. Because they feed on the nectar of small flowers, you can increase the population of this pest-punching good bug by planting plenty of flowers in the carrot and aster families (Apiaceae and Asteracae respectively).

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