Research Topics: Biological Control
About this Research:
Biological Control of Strawberry Guava in Hawaii
Strawberry Guava: Not All Green Is Good
Though it yields fruit and wood, strawberry guava is one of the most serious threats facing Hawaii's native forests.
Characteristics of Strawberry Guava
Strawberry guava (Psidium cattleianum) is native to southeastern Brazil. It commonly grows there in coastal plains and Atlantic forests up to about 4,000 feet (1200 m) in elevation and is occasionally cultivated as an ornamental fruit tree. In Brazil, strawberry guava typically ranges from 3 to 16 feet (1 to 5 m) in height, and its fruit is variable, yellow varieties being more common than red.
In 1825, strawberry guava was brought to Hawaii for its fruit and ornamental attributes. It is now common on all of the major Hawaiian Islands between sea level and 4,000 feet in elevation, especially in landscapes that receive moderate to high amounts of rainfall. The tree spreads by both shoots and seeds and grows fast in Hawaii, owing in part to the absence of the predators and diseases found in its native Brazil. Nonnative birds and pigs, which consume the fruit, also play a significant role in spreading strawberry guava to new areas within Hawaii.
Strawberry guava's fruit are edible and can be made into juice and other food products. Its smooth bark and glossy leaves also make it an appealing ornamental species. In Hawaii, it is also used as firewood and to smoke meat.
Effects of Strawberry Guava on Native Forests
Aerial view of a strawberry guava invasion. Photo courtesy of Carnegie Airborne Observatory.
Since its introduction in 1825, strawberry guava has become widespread, invading hundreds of thousands of acres of native forest across the Hawaiian Islands. Research shows that strawberry guava can ultimately invade almost half of the land area of Hawaii Island, degrading nearly 300,000 acres (120,000 ha) of conservation lands on that island alone. Remaining rainforests on other islands are similarly threatened.
Once established in native landscapes, strawberry guava forms impenetrable thickets that:
- Crowd out native plant species
- Break up natural areas
- Disrupt native animal communities
- Alter native ecosystem processes, like water production
- Provide refuge for alien fruit flies that are a major pest of Hawaiian agriculture.
Strawberry guava is particularly damaging in Hawaii because the tree grows aggressively without the natural "checking" power of the predators and diseases found in its native Brazil.
Because of its destructiveness to native Hawaiian forests, strawberry guava is recognized by scientists and land managers as one of Hawaii's worst invasive species. (Visit The Hawaii Conservation Alliance for more information on preserving native Hawaiian forests.) Although many people in Hawaii consider strawberry guava a familiar and useful plant, many also recognize the importance of curbing the growth and spread of strawberry guava for the long-term health and existence of native Hawaiian ecosystems, which provide a fuller suite of benefits to both people and the natural environment.
Frequently Asked Questions (FAQs)
Q. Why is strawberry guava a problem for Hawaiian forests?
A. Land managers, conservation groups, and state and federal agencies in Hawaii consider strawberry guava to be one of our most destructive invasive species. It is now common on all the major Hawaiian Islands between sea level and 4,000 feet (1200 m), infesting many thousands of acres of wet ohia and koa forest, where it forms dense thickets, replaces native plants, and destroys habitat for native birds and insects. Its prolific fruit production and sprouting ability allow it to spread rapidly, reach high densities, and overwhelm the ability of managers to bring it under control. Compared to healthy native forests, forests invaded by strawberry guava lose over one quarter more water to the atmosphere, robbing our streams and groundwater systems of this precious resource.
Q. Aren't there methods to control strawberry guava by hand?
A. Yes; however, only small patches can be managed in this way. For many years, forest managers have attempted to control strawberry guava by cutting stems and using herbicides. Over hundreds of acres, these methods become extremely expensive and difficult, especially on rough terrain and in areas only accessible by helicopter. Also, strawberry guava resprouts easily, so repeated treatments are needed over many years. Introducing a natural enemy of strawberry guava is expected to improve the effectiveness of other control methods by reducing the ability of strawberry guava to regenerate following treatment.
Q. Why was this insect chosen to control strawberry guava here in Hawaii?
A. Currently, strawberry guava has no natural enemies to keep it in check in Hawaii. The scale insect Tectococcus was selected after many years of research showed that it was both safe and effective. Studies in Brazil show that this insect feeds very specifically on strawberry guava, causing reduced growth and seed production. Extensive testing of over 80 native, commercial, and ornamental species demonstrates that Tectococcus will not affect any other species in Hawaii.
Q. What effects will this insect have on the strawberry guava plant itself?
A. Young Tectococcus scale insects settle and feed on newly sprouted leaves of strawberry guava and cause the leaves to form growths called galls. Because the plant puts energy into forming galls, it doesn't put as much energy into growth and fruit production. The presence of this natural enemy will reduce the vigor of strawberry guava, slowing its spread and making it less competitive against native plants. The scale will not kill strawberry guava. At the points of introduction of the scale, impacts on strawberry guava growth and reproduction should be noticeable within a few years. The insect is expected to spread gradually over a period of decades.
Q. How do you know this Brazilian scale will not hurt other species?
A. Observations in Brazil and rigorous testing demonstrate that this insect is adapted to feed only on strawberry guava and poses no threat to other plants in Hawaii. Strawberry guava's nearest relative in Hawaii, the agriculturally grown common guava (Psidium guajava), is also native to Brazil and is never infested with the scale there, even though the two guava species can be found growing side by side. Tectococcus has been known in Brazil for a century and never has been recorded as a pest of any agricultural or ornamental plant. Detailed studies over the last 15 years have shown that native Hawaiian and introduced plants in the same family as strawberry guava--ohia, mountain apple, and jaboticaba, among others--will not be affected by the scale.
Q. Isn't this insect likely to adapt to feed on other plants over time?
A. No. In its native range in Brazil, Tectococcus is exposed to a great diversity of plants including hundreds of species in the same family as strawberry guava, but the insect is found only on strawberry guava and one closely related plant found only in Brazil. The evidence from its native range indicates that this insect has not adapted to feed on other species in many thousands of years. Adaptation to feed on new plants is extremely unlikely for a gall-forming insect because it has such a close relationship with its host. Because Tectococcus is entirely dependent on its host, if strawberry guava populations gradually decline, the scale populations will decline as well.
Q. Won't strawberry guava be replaced by something worse?
A. This is unlikely. This natural enemy reduces the vigor of strawberry guava, but does not kill trees. There will be no widespread die-off of strawberry guava that might expose watersheds or open the forest to the spread of other invasive plants. Reduced vigor of strawberry guava should enable native species to grow and spread. By slowing regeneration of strawberry guava where it is removed by cutting, Tectococcus can enhance our ability to restore forests with native species.
Q. Don't native birds depend on the fruit?
A. No. nonnative birds, pigs, rats, and insects eat strawberry guava fruit, but native birds depend primarily on native Hawaiian plants and insects for their food. Strawberry guava destroys native forest habitats, reduces the abundance of native plants, and provides little in the way of insect food for birds. Because of its ability to degrade native forests, strawberry guava is considered one of the greatest threats to endangered forest birds on all the main Hawaiian Islands.
Q. How can I protect trees in my own yard from the Brazilian scale?
A. This scale insect has limited dispersal ability, so property owners are unlikely to see effects on their trees for many years. Homegrown strawberry guava trees could be protected by spraying organic horticultural oils commonly used on fruit trees to control scales and other pests.
Q. Many people use the wood and fruits of strawberry guava. How will those uses be affected?
A. The Brazilian scale cannot eliminate strawberry guava, only reduce its vigor and spread. Wood will continue to be abundant. Fruit should still be common when in season, although not superabundant in forest areas as it is now.
Click photos to enlarge.
Over time strawberry guava displaces native Hawaiian rainforest. These photos show a typical sequence from upper elevations that have not yet been overrun by strawberry guava to lower elevations where the native species have been severely impacted and, in the end, replaced completely. The sequence is along a trail in the Hawaii Experimental Tropical Forest, Laupahoehoe unit (Laupahoehoe Natural Area Reserve) on Hawaii Island. Photos by Christian Giardina, US Forest Service
[Click to enlarge first photo and scroll through to see progress of the invasion.]