Living creatures can be grouped as specialists or generalists based on their strategies for finding food and habitat: generalists thrive in a variety of conditions and eat a range of foods; specialists rely on a particular environmental condition or a food type. For example, specialist species like koalas only eat eucalyptus leaves, Kamehameha caterpillars rely on māmaki, and our iconic ʻāhinahina (silverswords) are found only in high-elevation alpine environments in Hawaiʻi.
When researchers look for a biological control agent – another living species that can be used to reduce a pest – they look for specialist species. This is part of an approach that began over 40 years ago with the goal of reducing unintended impacts. It works. Since 1975, over 50 different biological control agents have been brought into Hawaiʻi. “There have been no unpredicted non-target impacts in that time,” says Darcy Oishi, the Biological Control Section Chief at the Hawaiʻi Department of Agriculture. He oversees the efforts to find, test, and release biological control in Hawaiʻi.
“We worry most about specificity,” says Oishi. Host-specificity, the relationship between the pest and the natural enemy, is the creed of modern biological control. By seeking out species that have evolved to be dependent on the target host, there is less risk that the natural enemy will have unintended consequences.
Wiliwili are endemic to the Hawaii Islands and are a keystone species in the dryland forest. Photo courtesy of Forest and Kim Starr 
A tiny wasp hitchhiked to Hawai’i and found a home in Hawai’i’s wiliwili. The wasp caused the tree to form galls on the leaves and eventually fall off. Without leaves, trees begin to die. MISC file photo. 
Entomologists traveled to the pest wasps’ home range in search of a natural enemy that relied exclusively on the species. They found an even smaller Eurytomid wasp that parasitized the pest. MISC file photo. 
Darcy Oishi, HDOA, releases the Eurytomid wasp in Pu’u o Kali. After extensive testing to ensure its safety, the Eurytomid wasp was cleared for release in Hawai’i as a biological control. It’s proven effective at keeping the pest wasp in check, saving wiliwili. MISC file photo.
As new pests arrive in Hawaiʻi (and sometimes even before), the Hawaiʻi Department of Agriculture compiles lists of possible natural enemies, specialist species that could act to lessen the invasiveness and impacts of the pest in Hawaiʻi. Scientists comb through the scientific literature, call and visit natural history museums, and consult with entomologists and researchers worldwide, learning what species live in close association with the pest in its native habitat.
Exploratory entomologists then travel to the pest’s native range to collect candidates. Each potential natural enemy is studied further in its host range. After ruling out potential candidates, entomologists collect several species for additional testing.
Back in Hawaiʻi, the biological control agents are quarantined, raised and tested again for host specificity. In a process called no-choice testing, a potential agent is evaluated throughout its life stages to see if It will eat species it is likely to encounter in Hawaiʻi. For example, in introducing a biocontrol for a plant, the agent is isolated with other non-target plants to find out whether any unintended plants could be affected.
Researchers evaluate host specificity by looking at genetically related species as well as unrelated species that evolved similar characteristics to the target. For example, pasture-invading fireweed is in the aster family, but the phytoalkaloid – the toxic chemical that sickens animals – is what attracts one natural enemy, a caterpillar, to the plant. The caterpillar is tested to see if it can feed on related plants in the Aster family as well as those containing a similar toxic chemical.
Fireweed, Senecio madagascarensis, is a pasture invading pest that contains a chemical poisonous to horses and cattle. Photo by Forest and Kim Starr. 
The Senecio extensa caterpillar is attracted to the toxic chemical found in the invasive fireweed plant. To ensure host specificity, entomologists tested to see if the caterpillar would eat related Asters and other toxic plants. Photo by Forest and Kim Starr.
Until the 1970s, biocontrol agents were selected primarily based on the impact they would have on the pest, an approach that has unfortunately biased people against biological control today. Since then, though, rigorous selection and testing has gone into preventing unintended consequence. Another challenge is caused by misinformation; some invasive pests are inaccurately equated with biological control. For example, ʻōhiʻa rust, a leaf fungus that threatens our native ʻōhiʻa, decimated populations of rose apple, leading some to inaccurately assume it was an intentional introduction; instead, it was another of the thousands of unwanted pests that arrive in Hawaiʻi accidentally.
Modern biological control provides long-term, cost-effective, environmentally safe control of invasive species. It’s an essential tool in protecting Hawaiʻi from the impacts of widespread invasive species. Biological control does not eradicate a pest; it balances the equation to lessen the harm. The biological control for prickly pear cactus hasn’t eradicated it; the plant is still here, but instead of blanketing upcountry pastures it occurs in patches. Biological control saved the wiliwili trees of Hawaiʻi from decimation by a leaf gall wasp. The problem-causing wasp is still here but does less damage with the biological controls working to keep it contained.
Take time to learn more about biological control in Hawaiʻi and contribute to the decision-making process. Visit the Department of Land and Natural Resources biological control page at https://dlnr.hawaii.gov/hisc/info/biocontrol/
Lissa 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.
This article was originally published in the Maui News on May 9, 2020 as part of the Kia‘i Moku Column from the Maui Invasive Species Committee.
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