#Parasiteweek: Fly Attacks Caterpillar

August 12, 2016

Despite being the type of vegetable gardener who studies tomes on best practices, I have a hippy family that nurtures the welfare of tobacco hornworms (Manduca sexta). The big green caterpillars—considered major garden pests—feast and fatten like Henry VIII on the leaves, stems and fruit of the tomato plant. The caterpillars are masters of camouflage, blending into the dense, green foliage while clinging to the underside of leaves, and you often don't know that they're there until you spot their sizable dark droppings, the swift defoliation of your plant, or the mauled flesh of ripening tomatoes.

Caught in the act. Tobacco hornworm enjoying a black cherry tomato. Photo by: Candice Kim

You might consider the caterpillars, with their menacing red tail spike, unsympathetic garden inhabitants. The larvae, however, transform into the striking sphinx, or hawk, moth. Sphinx moths (family Sphingidae) are so large that species that hover while feeding on flower nectar have been mistaken for hummingbirds. Our local tobacco hornworms metamorphose into a mottled gray moth with an impressive four-plus-inches wingspan.

My family has raised numerous hornworms to their glorious adult form. Gently disengaged from our tomato plants, the caterpillars are given a new home in a small, pink-lidded terrarium, with a supply of fresh tomato plant branches and a moist bed of soil. Because they are mature caterpillars by the time we spot them and prepare their temporary plastic quarters, they are usually ready to pupate in the terrarium soil only a couple days after joining our household. Most have typically emerged weeks after pupating, but once we had a moth emerge ten months later. 

Our hand-raised sphinx moth, released after 10 months' pupation in a terrarium on top of the refrigerator. Photo by: Karen Klabin

Recently, I found a tobacco hornworm on the back of a tomato plant only because I could actually hear it in battle with a sarcophagid fly. The parasitic fly, unappealingly called a “flesh fly,” is ovoviviparous, meaning it deposits its progeny—maggots—in, among other places, the flesh of other creatures. The maggots then eat their way out of the reluctant host, thereby, in the case of the caterpillar, killing it (an event most gardeners would welcome). I watched the fly buzz and dart around the caterpillar, which squirted a dark liquid and twisted and seemingly snapped at the fly to try to keep it at bay. After a few minutes of battle, the dauntless fly landed successfully on the caterpillar, presumably depositing its larvae. I left the caterpillar on the tomato plant to fulfill its martyrdom as a maggot buffet.

The perp up close. The sarcophagid fly takes a break from attacking the hornworm. Photo by: Martin Schlageter

After returning from a short trip, I examined the tomato plant and failed to find the hornworm itself but did find the evidence of its diligent employ: a number of perfectly ripe tomatoes marred by gnawed and moldy flesh. The tomatoes weren't salvageable. And so the thought of the hornworm being parasitized by the sarcophagid fly did not break my heart. Nor those of my hens, who enjoyed the ruined tomatoes.

The ladies enjoying a hornworm-ravaged tomato. Photo by: Karen Klabin

(Posted by: Karen Klabin)

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