Psidium cattleianum — Maui Invasive Species Committee (MISC)

Lloyd Loope

Last month I began to present my point of view as a conservation scientist relevant to current “controversy” over biocontrol of strawberry guava (Psidium cattleianum), Hawaii’s most damaging invasive plant species. I touched upon the generally accepted theory that many highly invasive plant species, held in check by insects and pathogens in their home range, become unbeatable competitors in their introduced range where those insects and pathogens are lacking. Strawberry guava exemplifies the “enemy release” theory; it is attacked by virtually no insects or pathogens wherever it invades on many tropical islands of the world.

Though I’ve always supported the concept of biocontrol as a potential tool for Hawaii’s most widespread and most invasive plants, I recall a personal pessimism in the 1980s that biocontrol success might be unlikely in this case because guava is in the myrtle family (same as Hawaii’s ‘ōhi‘a) and commercially important common guava (Psidium guajava) is such a closely related sister species.

A strawberry guava tree in Curtiba, Brazil is heavily infested with the galls of the scale insect Tectococcus ovatus. This is the typical effect of the insect on the plant. It was a result of a naturally developing infestation of the insect in the plant’s native range. U.S. Forest Service photo

But state and federal natural resource agencies in Hawaii have managed to cobble together a remarkably stellar program to address the rampant invader. In 1991, the National Park Service and the University of Hawaii began to collaborate with the Federal University in Curitiba, Brazil, to identify and evaluate potential biocontrol agents. After 10 years of field observations and other testing working within the native ranges of both strawberry guava and common guava in Brazil researchers found 133 insect species in 12 orders and 80 families that fed on strawberry guava. Thirty-one of those species fed only on plants in the myrtle family, and five species fed only on strawberry guava and not at all on common guava. The collaborative program zeroed in on Tectococcus ovatus, a scale insect that feeds on the new growth, creating galls on the leaves of strawberry guava. This scale insect was the choice for more intensive work based on the relative severity of damage inflicted on P. cattleianum and the ease of handling it.

Tectococcus ovatus was brought to containment facilities in Hawaii and Florida, where strawberry guava is also invasive, for intensive experimental testing to ensure its safety as a biocontrol agent. Many related native and non-native plants were tested as potential host species. Strawberry guava is the only plant in Hawaii that this insect is able to feed on. Dr. Tracy Johnson of the U.S. Forest Service in Hilo has brought the program close to fruition over the past eight years.

The insect reduces the plant’s energy and nutrients available for growth and reproduction. Heavy infestation is expected to accelerate leaf drop and reduce fruit and seed production. Some fruit will be produced and the plants are not expected to die. If the scale insect thrives, it will level the playing field between the invasive strawberry guava and native forest species.

What other species in the ecosystem will be affected by the expected decline in strawberry guava fruit production? Feral pigs and a few species of non-native birds will lose a seasonal carbohydrate diet supplement during the September-December fruiting season. These same species are fully able to thrive in areas where there is no strawberry guava.

Hawaii is confronted by intractable invasive species problems, but by integrating prevention, early detection, rapid response, and biocontrol we can begin to address these problems. Biocontrol is an essential part of invasive plant management strategies in New Zealand, Australia, and the U.S. mainland. Florida has become a leader in biocontrol in the past decade. For example I recently learned that thanks to two insects introduced about 10 years ago as classical biocontrol agents, the notorious Melaleuca, or paper bark tree is no longer invasive in the Florida Everglades! My scientific opinion is that biodiversity conservation in Hawaii will not succeed without the enlightened use of biological control.

Lloyd Loope is a research scientist with the U.S. Geological Survey stationed at the Haleakala Field Station. He holds a doctorate in botany from Duke University and is an active Maui Invasive Species Committee member.

Originally published in the Maui News, October 12, 2008 as part of the Kia‘i Moku Column.
You can see all the articles in the Kia‘i Moku series http://www.hear.org/misc/mauinews/

The fruit if strawberry guava are eaten by birds that spread the tiny seeds. Photo by Forest & Kim Starr

When I came to Maui 28 years ago to work as a research biologist at Haleakala National Park, one of the first things I learned was that strawberry guava (Psidium cattleianum) was regarded as the most damaging invasive plant in the state. A year later, I was in Kipahulu Valley and experiencing the amazing guava fruiting and dispersal season. The 10-30 foot tall trees left a sea of red fruits that still lurks in my memory. There were dense, extensive guava thickets of tens of acres at the 2,000-3,000 ft elevation level in the valley. One could scarcely walk through; there were literally no other species present. These devastated areas contrasted sharply with the rich biodiversity in nearby areas with intact native understory vegetation.

A dense infestation of strawberry guava, or waiawī , in the Makawao Forest Reserve. Photo by Forest and Kim Starr

At that time, C.H. Diong, a graduate student at UH-Manoa, was putting the final touches on his Ph.D. thesis. Diong showed that feral pigs had been the primary agent spreading strawberry guava from the mouth of the valley upslope for the past 30 years or so. The pigs fed almost exclusively on guava fruits during September-December. The seeds pass through the pigs’ digestive tracts with no loss of viability. Fruiting was occurring only below about 3000 ft elevation but the seeds could germinate and grow much higher, up to over 4000 ft in the valley.

Years later, my colleague Dr. Art Medeiros did his Ph.D. thesis on the biology of weeds in Kipahulu Valley and East Maui. Art estimated that strawberry guava already occupies over one-third, or 8900 acres, of its potential area of dominance on East Maui. Although feral pigs had been eliminated and fenced from Kipahulu Valley since the late1980s, Art found that non-native birds, primarily the red-billed leiothrix, were also spreading strawberry guava seeds. Strawberry guava plants were still not fruiting above 3000 ft, but seeds planted experimentally (or those carried by birds or pigs) germinate at much higher elevations. Strawberry guava now reaches as high as 5300 ft on East Maui in at least one location.

Strawberry guava has infested the outlined area on Mahana Ridge, just to the east of Honokahua Valley in West Maui. Photo by Randy Bartlett.

Newly established guava plants can spread vegetatively by root sprouts. On all the major islands, nearly monotypic guava stands infest thousands of acres of moderately wet (mesic) and very wet forest. Strawberry guava is considered a serious threat to native forests due to its ability to invade relatively undisturbed areas and form thickets with dense mats of feeder roots. Dense invasion allows only about 5% of sunlight into the understory. The guava at first invades under native canopy, then expands and closes the mid-level canopy position, preventing the establishment and regeneration of all other species, including the dominant native forest plants – ohia and koa. Although it continues to expand into relatively pristine native forest areas, strawberry guava is now so widespread in Hawaii that its future impacts are expected to consist mainly of filling in areas creating monotypic stands.

As one of the founding members of the Maui Invasive Species Committee, I never considered taking on strawberry guava as a MISC target given its wide distribution and abundance and MISC’s limited resources. We always hoped for the eventual possibility of identifying an effective biological control.

A generally accepted theory of biological invasions is that certain plant species, held in check by insects and pathogens in their home range, become invasive in their introduced range where those insects and pathogens are lacking. Strawberry guava seems to fit the “enemy release” theory perfectly. In Hawaii, it is currently attacked by no insects or pathogens.

The first biocontrol researcher for this species, Dr. Charles Hodges, traveled to coastal Brazil in 1988 and reported that strawberry guava occurs not in dense stands but as scattered trees and rarely in small clumps. He noted that “few trees were observed that had not been attacked by at least one kind of insect,” and “often four or five types of insect damage could be identified on the same tree.” After 20 years of research to determine the best potential biocontrol agent, the scale insect Tectococcus ovatus has jumped through all the hoops but one – public opinion.

Lloyd Loope is a research scientist with the U.S. Geological Survey stationed at the Haleakala Field Station. He holds a doctorate in botany from Duke University and is an active member of the Maui Invasive Species Committee.

Originally published in the Maui News, September 14, 2008 as part of the Kia‘i Moku Column.
You can find all the articles in the Kia‘i Moku series http://www.hear.org/misc/mauinews/

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