Black caterpillar a promising sight in fight against fireweed

A new fuzzy black caterpillar is starting to show up across Maui – a promising omen for cattlemen and conservationists alike. The caterpillar is the larva of the Secusio extensa moth, a biological control for fireweed released by the Hawaii Department of Agriculture to control fireweed in March 2013.

Diana Crow, a horticulturalist with Ulupalakua Ranch, has been helping to raise and release Secusio moths on Maui. “For a while I wasn’t seeing them at all. Then things changed when the rains came. I started seeing them around lights. That was the first indication they were reproducing in the wild.”

Impacts on fireweed aren’t likely to come immediately. Established in the islands since at least the 1980s, fireweed now infests an estimated 850,000 acres statewide. According to Rob Curtiss, an entomologist with HDOA, it takes an average of eight caterpillars to kill one fireweed plant. “What will probably happen is that the population (of Secusio) will continue to grow and reach epidemic proportions, then we will see them start to defoliate fireweed.”

Secusio may be getting some help from another unwelcome pasture pest; the caterpillars are capable of living off cape ivy, a noxious weed related to fireweed, and containing the same toxic properties. “It’s inedible for livestock,” says Crow of the ivy. “We don’t want it. Plus, it’s a threat to higher elevation native forest.”

The moth’s broad appetite isn’t a surprise to state agriculture officials. Before Secusio was released, entomologists reared the caterpillars in captivity to test what plants they would eat. Confined in cages with 88 different kinds of plants (71 of which were related to fireweed), they found it only ate 6 different species, including cape ivy, and all were closely related to fireweed. Any of the plants it caused significant damage to is considered a weed in Hawaii.

Also known as German ivy, cape ivy is extremely invasive. Introduced at the turn of the century to Kona, cape ivy escaped cultivation and now threatens watersheds and native forests on the Big Island and Maui. It spreads quickly and once established can be extremely tenacious. Pat Bily of The Nature Conservancy has found several small populations in the Waikamoi preserve. Using herbicide to control a smothering vine would damage native vegetation so Bily removes plants by hand. Every leaf, stem and root have to be removed or the plant re-sprouts; eradicating a handful of small populations is taking Bily years.

“Any feeding on cape ivy, even if it was low, is a benefit,” explains Rob Curtiss. “Cape ivy certainly can be an alternate host. If populations of fireweed go down and there are populations of cape ivy in the area, they will sustain a larger population of Secusio, and we’ll get a better impact on fireweed.”

How do you know if you have them? The larval stage of Secusio is a fuzzy black caterpillar with an orange head and sometimes orange feet. It ranges in size from 1/8th inch when young to just over 1 inch before pupating into a moth. The moth is beige with brown mottling on the wings and about the size of an almond.

Are other plants on the menu for the Secusio moth? Seeing the caterpillar on other plants doesn’t mean the caterpillar is feeding on them. According to Curtiss, most feeding happens at night. The caterpillars often leave the host plants during the day to avoid potential predation.

People may also see them when the caterpillars are looking for a place to pupate, preferring a dark place like a garage or shed. “If you see them, leave them alone or take them outside,” advises Curtiss. The moths are nocturnal and attracted to light so leaving outside lights on all night may keep them close to houses rather than seeking out fireweed in adjacent pastures.

For more information about the Secusio moth, visit the HDOA website:

* Lissa Fox Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia’i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.