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Can Hawaiian carnivorous caterpillars attack invasive ants or vice versa? Shinji Sugiura1,2

1Department of Forest Entomology, Forestry and Forest Products Research Institute (FFPRI), 1 Matsunosato, Tsukuba, Ibaraki 305-8687, Japan 2Center for Conservation Research and Training, Pacific Biosciences Research Center, University of Hawaii, 3050 Maile Way, Gilmore 408, Honolulu, HI 96822, USA E-mail: ssugiura@affrc.go.jp The Hawaiian Islands have allowed insects to evolve unique habits, such as ambush predating in caterpillars1 and ground-dwelling in damselfly nymphs2, because the islands are isolated and originally lacked predators, such as ants and wasps3. However, many exotic species have been introduced to the islands through human activities, which has affected the native fauna3.

Ambush carnivorous caterpillars (Eupithecia spp.; Lepidoptera: Geometridae) provide a remarkable example of a feeding behaviour unique to the Hawaiian Islands1,4. More than 20 endemic species of Eupithecia have been recorded from the Hawaiian Islands1,4. While the larvae of two Eupithecia species are herbivores, the larvae of other species on the islands are obligate predators1,4. The larvae of carnivorous Eupithecia perch inconspicuously along leaf edges (Fig. 1a) and stems and seize arthropods that touch their posterior body section1. Their prey includes various arthropods species, such as springtails, drosophilid flies, cockroaches, crickets, psocids, moths, and leafhoppers1,4. Eupithecia larvae even attack predatory arthropods. For example, Montgomery4 reported that Eupithecia orichloris

(Meyrick) preyed on a braconid parasitoid. Although phytophagous Eupithecia are frequently parasitised by parasitic wasps and flies, the carnivorous species of Eupithecia are rarely parasitised, suggesting that carnivorous Eupithecia counterattack the parasitoids4,5. However, spiders may defend against Eupithecia attacks; one spider, Theridion grallator Simon, was attacked by E. orichloris, but the spider escaped from the E. orichloris by biting it4. Similarly, ants with strong mandibles may counterattack Eupithecia larvae4. However, it is unclear whether Eupithecia can attack ants.

Here, I examined whether E. orichloris can attack ants under laboratory conditions (21°C). An E. orichloris larva (body length, 19.3 mm) and workers of the invasive ant Pheidole megacephala (Fabricius) collected from Oahu Island in early June 2010 were used for the experiment. Pheidole megacephala has minor (body length 2 mm) and major (3.5 mm) forms of workers; major workers have larger heads with stronger mandibles than minor ones6. First, I used forceps to place a minor worker of P. megacephala on a leaf where E. orichloris

Figure 1. a An Eupithecia orichloris larva perching on a leaf; b E. orichloris feeding on a minor worker of Pheidole megacephala; c The body of the minor worker was torn apart in the E. orichloris attack (the arrow indicates the removed abdomen); d E. orichloris was counterattacked by a major worker of P. megacephala (the arrow indicates the biting by P. megacephala); e The major worker escaped from the E. orichloris attack; f E. orichloris lost a foreleg as a result of the bite by the major worker (arrow).

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perched. The E. orichloris was observed to bend the seized the ant the instant the ant touched its posterior body. The caterpillar seized the ant using its thoracic (elongated spiny) legs (Fig. 1b), and then used its mandibles to tear the ant body in two (Fig. 1c). Thus, E. orichloris can successfully attack the minor workers of P. megacephala. The next day, I similarly placed a major worker of P. megacephala on a leaf where E. orichloris perched. The E. orichloris was also observed to attack the ant the instant the ant touched its posterior end. However, the ant counterattacked (Fig. 1d) and was observed to bite one of the thoracic legs of E. orichloris (Fig. 1d) and ultimately escaped (Fig. 1e). The E. orichloris lost a foreleg and consequently lost its ability to seize prey (Fig. 1f). The caterpillar died 10 days later.

The Hawaiian Islands, which originally lacked ants, have recently been invaded by many exotic ant species7. The invasion of native vegetation by exotic ants has reduced the abundance of endemic arthropods8. For example, the invasive ants Linepithema humile (Mayr), Anoplolepis gracilipes (F. Smith), and P. megacephala have gradually been invading native forests where Eupithecia caterpillars are found. Although it remains unclear whether the invasive ants have reduced the population of Eupithecia caterpillars, my simple experiment suggests that the invasive ant P. megacephala may affect Eupithecia caterpillars.

ACKNOWLEDGEMENTS. I thank Vincent Costello and Brenden Holland for providing the caterpillar. The author was supported by a JSPS Postdoctoral Fellowship for Research Abroad. REFERENCES 1. Montgomery, S. L. GeoJournal 7, 549–556 (1983). 2. Zimmerman, E. C. Insects of Hawaii vol. 2:

Apterygota to Thysanoptera, University of Hawai‘i Press (1948).

3. Ziegler, A. C. Hawaiian Natural History, Ecology, and Evolution, University of Hawai‘i Press (2002).

4. Montgomery, S. L. Carnivorous caterpillars: the systematics, behavior, biogeography and conservation of Eupithecia Curtis (Lepidoptera: Geometridae) in the Hawaiian Islands, Ph.D. Dissertation, University of Hawaii (1984).

5. Henneman, M. L. and Memmott, J. Science 293, 1314–1316 (2001).

6. Wetterer, J. K. Pacific Science 61, 4437-456 (2007) 7. Krushelnycky, P. D., Loope, L. L. and Reimer, N. J.

Proc. Entomol. Soc. 37, 1–25 (2005) 8. Cole, F. R., Medeiros, A. C., Loope, L. L. and Zuehlke,

W. W. Ecology 73, 1313–1322 (1992).

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