This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Tachinid Fly (Diptera)
There are more than 1,300 North American species of parasitic flies. Most resemble robust houseflies, but with short, bristly hairs on their rear ends. Tachinid flies are gruesome parasites of other insects. They employ a variety of tactics, with a common practice being to glue an egg onto another insect, so that the tachinid fly maggot can consume it as food. Other species of tachinid fly may lay mini-eggs on foliage being eaten by insects, which hatch in the insects’ bellies, or they may inject their eggs directly into another insect’s body with a sharp ovipositor.
Egg and larval development happen quickly in tachinid flies. Indeed, a quirk of many species is their ability to lay eggs that hatch almost immediately, or even before they are laid. Many tachinid fly species pass from the early stages to adulthood in just three to four weeks. If the host also moves through life stages quickly, several generations can be produced in one garden season. At the end of the season, some tachinid flies overwinter in leaf litter, while others spend winter inside the pupal cases of their victims.
Tachinid Fly Diet
Tachinid flies feed on caterpillars, beetles, bugs, earwigs and grasshoppers. Garden pests likely to be impacted by tachinid flies include gypsy moths, cabbage worms, Japanese beetles, armyworms, cutworms, sawflies, codling moths, peach twig borers, pink bollworms, tent caterpillars, leafrollers and squash bugs. Adult tachinid flies feed on nectar, pollen and honeydew.
How to Attract Tachinid Flies to Your Organic Garden
Tachinid flies can easily feed on the nectar of Queen Anne’s lace and other members of the carrot family. Attract tachinid flies by growing plants that bear umbels of flat florets, including carrots, cilantro, dill, coriander, buckwheat and sweet clover. Anise hyssop (Agastache) is also a favorite of tachinid flies.
More information about tachinid flies is available from the University of California, North American Dipterists Society, and the Agricultural Research Service.
At MOTHER EARTH NEWS, we are dedicated to conserving our planet's natural resources while helping you conserve your financial resources. You'll find tips for slashing heating bills, growing fresh, natural produce at home, and more. That's why we want you to save money and trees by subscribing through our earth-friendly automatic renewal savings plan. By paying with a credit card, you save an additional $5 and get 6 issues of MOTHER EARTH NEWS for only $12.95 (USA only). 
Jules, thank you for the fly trap info. I am also going to relocate the cilantro and Agastache that I have growing in the backyard near the milkweed. Maybe with those changes, we will see an improvement. Thus, far this year, we are at a 100% failure rate for any Monarch caterpillars successfully pupating into adults. We've seen the tell tale strings of the tachinid larvae. I will be sure to destroy any dead caterpillars by immersing them in alcohol prior to disposing of their poor little bodies. We are devastated at the loss rate this year.
Jules, thank you so much for a way of slowing down the tachinid flies. We've been devastated that none of our Monarch caterpillars are making it into the chrysalis stage and have the tell-tale strings hanging from their bodies just after they attach to begin forming the chrysalis. I will be moving the cilantro plants to a different location if not outright getting rid of them. I also see that the Agastache will need to be relocated as well. Grrrrr.
I have found that if you heat up 4 cups of water, 2 cups of sugar, 1/4 cup honey, then Add 3 tbls of boraxo pour into reusable fly traps, you wonāt kill all of the tachitflies killing your monarchs, but you will slow down the population of the little jack asses! I have 10 traps on my 1/4 acre property and dump and refill every 2 months. So far only 20% of my Monarchs make it. There is a misnomer that they canāt infect eggs but that is not true! They can.
I realize that Tachinid flies fall into the "beneficial pest" category, but not in my case. Tachinid flies are killing 100% of the Monarch butterfly caterpillars in our butterfly garden. Are there any natural means to discourage them from parisitizing our Monarchs?