Insect Invasion to Guam
For an organism to be considered “invasive” it must meet some general requirements. The US Department of Agriculture, under Executive Order No. 13112, defines what makes a species invasive:
- non-native (or alien) to the ecosystem under consideration
- introduction causes, or is likely to cause, economic or environmental harm or harm to human health.
Islands, in particular, are susceptible to invasive species because their biological communities evolved in relative geographic isolation. In the distant past, vast mountains and oceans proved sufficient to prevent the migration of even the hardiest of species. Humans have proven able to cross these great distances on ships and planes, sometimes taking hitchhikers on the journey.
Guam, in particular, has become infamous since the accidental introduction of the brown treesnake in the 1940s, and the devastating effect its introduction has had on the native bird population. Perhaps if the native forest bird population was still thriving on Guam, the insect population would be better controlled.
Although many people who have never visited Guam know of the brown treesnake, little is known to outsiders about the invasive insects. Here are some highlights of the species that have created some of the worst environmental and economic impacts on Guam.
Coconut Rhinoceros Beetle
The coconut rhinoceros beetle (Oryctes rhinoceros or CRB) is native to Asia, from India to Indonesia. Recently, CRBs were introduced through a ship transport of construction materials to Guam. It is difficult to say which company introduced the CRB because any company you question may say it wasn’t them. In 2007, the beetles were first identified to be established in the village of Tumon. Since Typhoon Dolphin in 2015, the rhino beetle populations have spread all around the island. The results from Typhoon Dolphin caused many coconut trees to fall and decay, which provided a number of suitable breeding grounds for the beetles to reproduce.
Figure 1. CRB lifecycle showing each timespan from egg to adult stage
The CRB goes through a complete life cycle from an egg to larvae to pupa and then into an adult (Figure 1). The larvae (called “grubs”) resemble large, fat, white worms that range up to 10 centimeters long and are found in shallow soil. The larvae body forms a C shape with three pairs of segmented legs on both sides of the body. The adult beetles range from 3.0 to 6.3 centimeters long and are either black or brown, with a prominent horn on their head. Adult male and female beetles can be easily distinguished by identifying their abdomens and the size of their single predominant horn. The tip of the abdomen of an adult female has furry brownish-red hairs which the males do not possess. The adult male beetle horn size is occasionally longer than that of the adult female. The coconut rhinoceros beetles horns are used for leverage and for burrowing into coconut palm crowns.
CRBs feed on the sweet sap in the heart of the palm. After feeding on sap, the beetles search for a suitable breeding ground. After breeding is complete, the female lays her eggs in a dead standing coconut tree or in piles of decaying organic matter. The eggs will hatch into grubs and which will feed on dead coconut trees and green waste piles with high organic matter content and decaying plant matter. After continuous eating for 60 days, the grubs metamorphoses, where it develops into a pupa for up to one month and then metamorphoses again into an adult beetle.
Flame Tree Looper
Figure 2. Flame Tree Looper caterpillars. The moths are preserved in 100% ethanol, which has changed its natural color. (Credit: Kaleb Leon Guerrero).
The Flame Tree Looper (Pericyma cruegeri) is a moth found in Southeast Asia, Australia and several Pacific Islands in the tropical and subtropical zones. Flame Tree Loopers (Figure 2) were first reported on Guam in 1971. Other reports of the looper have been found in Japan in 1986, on Ishigaki Island and Okinawa Island in 2000. P. cruegeri is a serious pest on the royal poinciana (Delonix regia), yellow flame tree (Peltophorum pterocarpum), and has been found on wild tantan (Desmanthus virgatus), golden shower tree (Cassia fistula), and tangan-tangan (Leucaena leucocephala). On the royal poinciana, they feed on the foliage of the tree and are abundant during the wet season. The caterpillars strip the trees of most of their forage, triggering the creation of smaller leaves, death of small branches, and desynchronized flowering.
Flame tree looper eggs are yellowish or bluish-green are laid singly on the flame trees’ leaflets. The eggs then hatch within two to three days and the caterpillars take at least one month to mature. Once maturity is achieved, the caterpillar pupates on the tree in a cocoon made of webbing and leaves. Then the moth emerges in 12 days. The head of the caterpillar is distinctively large green with wavy white lines along the sides and top of its body.
The adult moths are dark grey in color, with a wingspan of 3.5 to 4.0 cm. Within the same sex they show minor differences in variation of color. Male and female sex can be determined by examining the antennae and hair growth on the limbs. The male’s antennae are thickened for approximately two-thirds of their length, while the female’s antennae are thin, simple and straight. The hairs on the limbs of males are in a dense profusion and are elongated, while females’ hairs are shorter and not as tightly clustered.
Asian Cycad Scale Insect
Figure 3. Left: A healthy cycad habitat on Guam prior to CAS invasion. Right: Widespread plant death caused by CAS. (Photo credit: Thomas Marler).
Native to Thailand and some parts of China, the armored cycad-specific scale insect (Aulacaspis yasumatsui or CAS) was first detected on Guam in 2003. Since that time, the formerly most abundant tree species, Cycas micronesica, has declined by an estimated 90 percent island-wide (Figure 3). First noticed on ornamental Cycas revoluta plants in Tumon, a failure to quickly isolate the initial infestation led to the armored scale spreading to nearby forests where C. micronesica plants (locally known as fadang) were abundant. Upon finding the vast forests of fadang on Guam, it must have seemed like an all-you-can-eat buffet for the insects. Fadang is the only native host plant for this armored scale insect. Feeding on all parts of the plant and being very small (less than 5 mm) has made controlling this alien herbivore increasingly difficult.
In its native range, this scale insect is a natural pest of various cycad species. Throughout their native range, their populations are kept in check by various predatory insects. On Guam however, these predators are not present.
Figure 4: The parasitic wasp, Coccobius fulvus. Left: adult form of the wasp; Right: Close up of exit holes of the wasp larvae from
Biological control efforts to control CAS have been met with mixed results. Two parasitoid wasp species, Coccobius fulvus and Aphytis lignanensis, failed to establish on Guam when introduced in 2005 (Figure 4). The predatory lady beetle, Rhyzobius lophanthae, was also introduced in 2005 and readily established throughout Guam (Figure 5). The lady beetle has helped reduce mortality from CAS herbivory but has not been effective in protecting seedlings and juvenile plants.
As a result, the once iconic fadang has become a threatened species, and current models predict extirpation in the wild by the year 2019. Until the ongoing herbivory by CAS is under control, reintroductions of C. micronesica will be futile. Off island collections of C. micronesica exist which can be utilized for future reintroductions.
Figure 5: larvae and adult of Rhyzobius lophanthae. (Photo credit: Aubrey Moore)
Cycad Blue Butterfly
Another alien herbivore has also added to the ongoing assault of the cycad trees. The Cycad blue butterfly (Chilades pandava) is native to India, Sri Lanka, Myanmar, United Arab Emirates, Indochina, Peninsular Malaysia, Singapore, Taiwan, Java, Sumatra and the Philippines. This butterfly was first detected on Guam in 2005. This lycaenid butterfly is a specialist herbivore of Cycas plants. Although still harmful to C. micronesica, herbivory from this pest is not fatal. As with CAS, C. micronesica is the only native host on Guam for this butterfly, making it a significant pest even if it is not fatal.
On the upper side of the wings, the male is blue with thin black borders on both wings and a black spot on the corner of the hindwing (a “tornal” spot). The female is a paler blue with broad borders on the forewing, a series of submarginal spots on the hindwing with the second spot crowned in orange. The female oviposits on young emerging growth of various Cycas species. After two days, the eggs emerge as caterpillars and consume leaf tissue on the soft and emerging leaflets (Figure 6) As a result, the plant is left only with the rachis.
Figure 6: Damage to Cycas sp. leaflets from Chilades pandava (photo credit: Benjamin Deloso)
In addition, several ant species existed on Guam prior to the butterfly’s invasion. These ants work together with caterpillars by protecting the caterpillars from would be predators. In return for protection, the ants receive a nectar reward from the caterpillars, thus adding to the assaults on Guam’s native cycad. Biological control of the Cycad Blue butterfly has not been attempted to date due to concerns over native butterfly species.
One of Guam’s smaller invasive species is the Asian Citrus Psyllid (Diaphorina citri) , which is native to Asia (hence its name). The psyllids (Figure 7) are pests to citrus plants. They are most noted for carrying a disease called the Citrus Greening Virus. The Citrus Greening Virus causes Guam’s citrus trees to lose color, form misshapen fruit, and to die in some cases. Since their introduction, the psyllid population has posed a threat to Guam’s citrus trees.
Figure 7: The Asian Citrus Psyllid (photo credit: Kaleb Leon Guerrero)
The Asian Citrus Psyllid was first found on Guam in 2008 by R.K Campbell. The psyllids may have been introduced by infested plant material, which has become a popular trend amongst invasive species. Psyllids prefer temperatures around 80 Farenheit, which makes Guam’s climate ideal for their population to increase. This increase in psyllid population may lead to the spread of the Citrus Greening Virus.
Two common ways to eradicate agricultural pests are the use of pesticides and biological control agents. Pesticides are more commonly used on citrus farms because of their ease of application over numerous trees. Although they are easy to use, pesticides are costly and may be dangerous. A more cost-effective way to manage Guam’s psyllid population is using biocontrol insects. There are five beetles, brown-lacewing larvae, and other insects that eat psyllids at different stages of their lives. To prevent the spread of this invasive insect, Guam needs to implement a combination of control methods.
Greater Banded Hornet
Figure 8: Adult Greater Banded Hornet (Vespa tropica). (Photo credit: Catherine Ngyuen)
A relatively new invasive insect is the Greater Banded Hornet (Vespa tropica). It is commonly found in Southeast Asia. The hornet was found on Guam on 12 July 2016 by Christopher Rosario, a University of Guam student. The Greater Banded Hornet (Figure 8) is characterized by a single large band, that wraps around the abdomen of the hornet. It is not every “big bee” that can be seen flying around your home.
Since its introduction to Guam, hornet attacks have been a public problem. In August of 2016, The Guam Daily Post wrote an article about firefighters being attacked by hornets. In this article, they describe how the firefighters were attacked and hospitalized during a dog rescue.
Figure 9: Map of Guam’s Greater Banded Hornet Sightings. (Photo credit: Christopher Rosario)
This kind of attack is not uncommon, as hornets are known to attack when their nest is disturbed. The Greater Banded Hornet also attacks bees and wasps. The hornets have been observed catching honey bee foragers, by Guam’s beekeepers.
Because Guam is small and lacks natural predators of the hornet, the hornets have been able to establish their population on Guam. The Greater Banded Hornet can travel miles away from their hive to forage. This ability to travel great distances has resulted in multiple sightings of the hornets (refer to Figure 9). Since their establishment, the hornet has been sighted in the northern part of Guam. If you have seen this invasive species, please call (671) 735-2001, or email Dr. Ross Miller at [email protected]
Little Fire Ants
Little fire ant, (Wasmannia auropunctata Rodger) (LFA), is a species of ant native to Central and South America but has now spread to parts of Africa, North America, Puerto Rico, Israel, Cuba, and now several Pacific Island groups including Guam. LFAs (Figure 10) were first detected in a karst limestone forest adjacent to a green waste landfill on Guam ( Latitude 13.541 N Longitude 144.908 E) in November of 2011. Since then this aggressive ant has spread to all parts of the island.
Figure 10. The Little Fire Ant, Wasmannia auropunctata. Left: Close up of the ant taken under a dissecting microscope. Right: Actual size of adult LFAs, with an insect pin provided for scale. (Photo credit: Kaleb Leon Guerrero)
The devastating effects that LFA has had on agriculture and local forest ecosystems outside of its native range are also likely to occur on Guam. Unfortunately, LFA has now spread all over Guam and infestations are particularly heavy in areas close to streams. The sting of LFA is very painful and those who may be allergic to stings are at particular risk. The sting of LFA may also cause blindness in indigenous and domesticated animals. Applying gelatin-based matrices containing toxicants has proven to be somewhat effective at controlling the LFA. Recent efforts to eradicate LFA using peanut butter and techniques developed by labs in Hawaii are currently being tested.
Benjamin Deloso is a graduate student majoring in Environmental Science at the University of Guam. He holds a B.A. in Biology with a minor in Psychology from the University of North Carolina Wilmington. During his time in college he became interested in botany and horticulture, and became enamored with a particular group of plants called cycads. Originally from Guam, his horticulture experience and botanical exploits led him to return to Guam to study under cycad expert and aficionado Dr. Thomas Marler. Since arriving at UOG in 2016 he has designed and maintained a cycad teaching garden located near the Agriculture and Life Science building as well as conducted fieldwork in cycad habitats in Yap, Palau, Rota, and the Philippines. Grateful for the opportunities to study cycads in the tropics, Benjamin hopes to become a world authority on cycads in the future. He wrote the sections on cycad-specific insect pests.
Chieriel Desamito is a graduate student majoring in Sustainable Agriculture, Food and Natural Resource (SAFNR) at the University of Guam. He holds a Tropical Agriculture Applied degree from the University of Guam in 2016. During his time in college, he took an interest in Soil Science after taking classes regarding soil health and fertility. He is now currently working as a Research Associate for the UOG soil laboratory and as a teaching assistant for Dr. Golabi’s AG380 lab (Fundamentals of Soil Science). He is currently conducting research with Dr. Golabi on the effects of biochar, compost, and fertilizer application as a soil amendment, in order to promote agricultural sustainability and reduce greenhouse gas emissions from intensive tillage practices. He hopes to have more research opportunities with Dr. Golabi and expand his knowledge on Guam’s soils. He wrote the sections on CRB and Flame Tree Loopers.
Kaleb Leon Guerrero graduated with a Research in Tropical Agriculture degree from the University of Guam in 2017. He is currently working towards his masters degree in Sustainable Agriculture, Family Science, and Nutrition at the University of Guam. Currently, he is working at the University of Guam as a Research Associate for the entomology lab, and an adjunct professor for the BI-100L class. During his time internship at UOG in 2008, he became interested in the field of entomology. Since his senior year in college, he has conducted research in entomology under Dr. Ross Miller. He has conducted field work on Guam, Saipan, Tinian, and Rota. Grateful for the opportunities his work in entomology has presented, he wishes to further his knowledge on different species of insects and conduct more field work in the future. He wrote the sections on Psyllids and the Greater Banded Hornet.
These authors were beginning graduate students in 2018 taking a course in scientific writing at the University of Guam. This article was assigned to provide the student with practice in communicating science to non-scientists. The student chose the topic which is related either to their thesis project or work experience. The instructor in the course is Dr. Laurie Raymundo, a UOG Marine Laboratory faculty member.
For further reading
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Bové JM., Huanglongbing: a destructive, newly-emerging, century-old disease of citrus, J. Plant Pathol. 88:7–37, 2006.
Campbell R.K., Incursion of Citrus Psyllid in Guam. Pest Alert, 40: 1727-8473, 2008.
Doane RW, How Oryctes rhinoceros, a dynastid beetle, uses its horn, Science, New Series 38: 883, 1913.
Dornberg, M., “Featured Creatures: Oryctes Rhinoceros. Retrieved 4 September 2018. http://entnemdept.ufl.edu/creatures/orn/palms/Oryctes_rhinoceros.htm
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Liu Y. H., & Tsai, J. H., Effects of temperature on biology and life table parameters of the Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae), Annals of applied biology, 137(3), 201-206, 2000.
Marler, T. and R. Muniappan, Pests of Cycas micronesica leaf, stem, and male reproductive tissues with notes on current threat status, Micronesia 39(1):1–9, 2006.
Marler, T. and J.H., Lawrence, Demography of Cycas micronesica on Guam following introduction of the armored scale Aulacaspis yasumatsui, J. Trop. Ecol. 28:233 242, 2012.
Marler, T. E., R. Miller and A. Moore, Vertical stratification of predation on Aulacaspis yasumatsui infesting Cycas micronesica seedlings, HortScience 48(1):60–62, 2013.
McCaffrey, S. & K. Walker, Greater banded hornet (Vespa tropica), 2012.
Michaud J. P., Biological control of Asian Citrus Psyllid, Diaphorina Citri (Hemiptera: Psyllidae) in Florida: A preliminary Report, 2002.
Rosario, C. A., Moore A., Miller R.H., Greater Banded Hornet Vespa tropica (Hymenoptera: Vespidae. Guam New Invasive Species Alert No. 2016-01, 2016.