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High Park Caterpillar Survey: Citizen Science When the Sun Goes Down

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by Richard Aaron

Each summer, the High Park Caterpillar Survey seeks volunteers to help document caterpillar species and contribute to community science. For more information and to apply, please visit their website.

As the sun dips below the horizon, High Park’s “night shift” stirs to life. Think owls, bats, slugs, snails and more. But of all the nocturnal groups of organisms in High Park, none come close to rivalling the diversity of moths.

The annual High Park public moth night is something every nature lover should experience at least once. Having attended multiple times, I began to wonder how many species of moths were in the park, which led me to start the High Park Moth Study. Over five years, from 2016-2020, our devoted group of moth enthusiasts – aka the High Park Mothia – held dozens of nighttime sessions each year. We set ourselves a very ambitious goal: to record 1,000 species, not knowing if this was even achievable. But achieve it we did, midway through our final season. The moth study ultimately went on to record 1,081 species, although the park undoubtedly contains even more species. The study’s “15 minutes of fame” occurred when we were featured in the Toronto Star.

During the moth sessions, we dabbled with using UV (ultraviolet) flashlights to search for caterpillars, in the process adding several moth species to the park list. The intrigue of searching for caterpillars with these specialized flashlights is what inspired me to launch the High Park Caterpillar Survey in 2020.

A green and yellow caterpillar with a brown and yellow-spotted head on a leaf.
Silver-spotted Skipper. Credit: Richard Aaron
A green caterpillar with black stripes sitting on a green leaf.
Abbott's Sphinx. Credit: Richard Aaron

More Species Than You Can Shake A Twig At

As most know, caterpillars are the larval stage of both moths and butterflies. In addition to nearly 1,100 species of moths, at least 74 butterfly species have been found in High Park since the 1980s, and 40 or more of them breed here annually. Assuming  that a good portion of the moth species recorded for High Park also breed there, that means the park plays host to a mind-boggling variety of moth and butterfly caterpillars.


While more than half a dozen techniques have been devised to search for caterpillars, the High Park Caterpillar Survey primarily uses UV flashlights. Many caterpillar species fluoresce when UV light is shined on them, making them visually stand out from whatever background they are on (leaves, bark, wood, etc.). This fluorescence is fairly weak and thus more difficult to see in daylight, which is why survey sessions take place after sundown.

Contributing to Community Science

Besides being personally rewarding, the survey also contributes to citizen science by:

  • finding species that weren’t recorded by the High Park Moth Study
  • educating volunteers about caterpillars and their ecological needs so that they in turn can educate others
  • helping participants develop an understanding of how caterpillar abundance and diversity are impacted by factors such as invasive plant species, artificial illumination, and habitat fragmentation and degradation
  • collecting long-term data that could provide insights into population patterns over time

Triple Digit Success

Since its founding in 2020, the caterpillar survey has recorded 120 species, consisting of 11 butterfly species and 109 moth species. On our nocturnal rambles, we also encounter a range of other fluorescing organisms, including slugs, snails, grasshoppers, cicadas, spiders, fungi, lichens, mosses, and various parts of grasses, wildflowers, trees, and shrubs.

Get Involved!

Itching to increase your IQ (Insect Quotient)? The survey welcomes new volunteers – no experience necessary. The season typically runs from late May to early October. By becoming part of the survey, you too can start experiencing the wonder and enchantment of searching for caterpillars after dark.

Learn More: High Park Caterpillar Survey (Includes a photo gallery of common caterpillars)

Event more photos: Caterpillars of High Park Gallery

The Intriguing World of Caterpillars

The more time spent learning about the natural history and ecology of caterpillars, the more fascinating these creatures become. Their diverse feeding strategies and various means of self-defence from predators, and the myriad ways that plants try to avoid being consumed by caterpillars, could each fill a stimulating multi-hour lecture.

The physical features of caterpillars are equally compelling. During the survey, we have encountered species large and small (ranging in length from over 9 cm to less than 1 cm), in a wide variety of shapes, colours, patterns and markings. One of the more intriguing types of markings we have observed is eyespots. It is believed that eyespots are intended to fool avian predators into thinking the caterpillar is actually a threatening creature with big eyes and thus should be avoided, although field experiments with mock specimens have shown mixed results. As well, some caterpillars sport fringes, spines, horns, bumps, ridges, or hairs of various sorts. Then there’s the osmeterium, a special organ that a few High Park caterpillars, such as the black swallowtail, possess. The osmeterium resembles a forked snake tongue and is normally hidden within the body. It will only be extended if the caterpillar is disturbed, releasing a chemical repellent containing a foul smell to repel predators. Not to worry though, the repellent is harmless to us humans.