Arcte coerula (Guenee, 1852)

Au, Michelle G. & Wright, Mark G., 2022, Ramie Moth, Arcte coerula (Lepidoptera: Noctuidae): A New Invasive Pest in Hawaii on Endemic Plants, Proceedings of the Hawaiian Entomological Society 54, pp. 63-76 : 64-73

Arcte coerula is native to Southeast Asia.

It has since spread into other parts of Asia including Japan, India, and Sri Lanka as well as

Oceania including Australia, Fiji, and Papua New Guinea (Jackson and Mua 2016). In Hawaii , A. coerula has been recorded feeding in both wild and cultivated populations of māmaki on Maui and Hawaii island . The known distribution on Maui is scattered across the island in a range of habitats ( Fig. 1a ). In November of 2019, A. coerula was detected in the Puna District of Hawaii island . As of January 2022, this pest has been detected from the Kaʻū District and up north to the Hilo and Hāmākua Districts. ( Fig. 1b ). Larvae have been recorded in residential areas, māmaki farms, and native forests.

Host range. Larvae are known to feed and reproduce on a range of host plants in the nettle family ( Urticaceae ). In its native range of Southeast Asia, A. coerula larvae are found on various ramie species ( Boehmeria spp. ), which are commonly used as a natural fiber crop (Ide 2006). However, it has also been reported feeding on other Urticaceae genera including Cypholophus Weddell , Debregeasia Gaudich , Girardinia Gaudich , and Pipturus Weddell (Jackson and Mua 2016) . Other authors have noted larvae feeding on Vitis sp. Linnaeus ( Vitaceae ) and Trema tomentosa (Roxburgh) H. Hara ( Cannabaceae ) ( Robinson et al. 2010). Adults will sometimes feed on tree sap and decaying or overripe fruit such as bananas (Jackson and Mua 2016).

Biology and description. Eggs. Eggs are clear-white, circular, about 1 mm in diameter, and usually laid singly on the underside of leaves ( Fig. 2a ). In some instances, multiple eggs may be laid on a single leaf. In general, females will lay an average of five to ten eggs per plant. However, we have found up to 75 eggs on a single tree, indicating that the females may egg dump during the end of their life cycle. There have been several observed occurrences of a female laying an egg mass with as many as 220 eggs on the

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underside of a leaf ( Fig. 2b ). It takes less than a week for eggs to hatch.

Larvae. Early instar larvae are green and white ( Fig. 3 ) and may be mistaken for the native Kamehameha butterfly, Vanessa tameamea (Escholtz 1821) ( Fig. 4a ) or the endemic moth Udea stellata (Butler 1883) ( Lepidoptera : Crambidae ) ( Fig. 5a ) which also use māmaki as a host plant. We have also found tomato looper, Chrysodeixis chalcites (Esper 1789) ( Lepidoptera : Noctuidae ) ( Fig. 6 ) feeding on māmaki. However, Arcte coerula can be distinguished from these other Lepidopterans by the presence of black dotted markings on the side of its body, thin white setae or hairs, and its defensive behaviors described in the biology and behavior section below. As they develop, A. coerula larvae become more distinctive, with vibrant yellow and black patterning and bright orange-red spots on their sides. There are two color morphs of A. coerula that develop in response to varying population densities which impact the production of a juvenile hormone (JH) analogue ( Ikemoto 1984). The black bands on the larva’s body are narrow under solitary conditions, creating a yellow morph larva ( Fig. 7a ), and conspicuously widen under crowded conditions, creating a black morph ( Fig. 7b ) ( Ikemoto 1984). Most larvae collected in Hawaii are the black morph indicating gregarious larval behavior.

First instar A. coerula larvae emerge at about 2 mm in length and grow rapidly through five instars, up to 100 mm before pupating. Although previous studies have indicated six larval instars ( Ikemoto 1984, Ide 2006), specimens collected in Hawaii typically had only five instars, with occasional larvae going through six or seven molts. Head capsule size was measured after each molt using a microscope micrometer from captive larvae reared from eggs, reported here as mean head capsule width and range: 1 st instar: x = 0.65 mm, 0.45–1.10 mm, (n = 25); 2 nd instar: x = 1.26 mm, 0.75–2.00 mm, (n = 19); 3 rd instar: x = 2.00 mm, 1.30–2.80 mm, (n = 17); 4 th instar: x = 3.12 mm, 2.25–3.85 mm, (n = 15); 5 th instar: x = 5.3 6 mm, 5.00–6.00 mm, (n = 9). Larval duration averaged about a month from egg emergence to pupation, but duration was variable.

Pupa. Larvae pupate in the soil or at the base of the plant in leaf litter. Pupae are an average of 28 mm, 25–39 mm, (n = 20) in length ( Fig. 8 ) and take about 13 to 25 days to emerge. Variation in emergence time is likely due to seasonal temperature changes.

Adults. Adults are large moths with a forewing length (FWL) ranging from 22 mm to 36 mm (x = 30 mm; n = 20). They have a dark brown head and dark brown forewings with black markings, speckles of silvery-blue, and scalloped termen or wing edges ( Fig. 9 ). Hindwings have distinctive bright silvery-bluish markings which distinguish them from the similarly large black witch moth Ascalapha odorata (Linnaeus 1758) ( Lepidoptera : Erebidae ) that is also found in Hawaii. Adults reared in captivity had a short lifespan, living no more than a week and sometimes feeding on decaying fruit and sugar water in the cages. There was approximately a 1: 1 male to female ratio. Sex was determined during the pupal stage based on the genital opening on the ventral side of the abdominal segments ( Lin et al. 2020).

Biology and behavior. In Southern Japan where ramie plants ( Boehmeria nipononivea Koidzumi ) are grown, A. coerula is multivoltine with populations appearing in the spring and fall with densities peaking in August and September ( Ide 2005). Field surveys on Maui and Hawaii island have also shown A. coerula to be multivoltine; however, populations peaked in the spring, from March to May, and decreased dramatically in the summer. Low-density populations of A. coerula can be found throughout the year in Hawaii. Arcte coerula larvae can be distinguished from endemic species on māmaki by their active movements and aggressive defensive behavior. Early instar larvae were observed defensively dropping from leaves when disturbed and moving quickly between leaves on the plant. Arcte coerula does not produce any protective structures, unlike the leaf folding behavior seen in Vanessa tameamea ( Fig. 4b ) and silk coverings made by Udea stellata ( Fig. 5b ). In later instars, Arcte coerula displays a distinctive, aggressive, defensive behavior when disturbed or threatened. The larva rears up its head and legs, thrashes around, and regurgitates a green liquid to defend itself. Similar behavior was also reported by Jackson and Mua (2016).

Damage. Arcte coerula larvae will feed on both young and mature leaves of host plants. Early instars create small holes on the leaves as they feed ( Fig. 10a ). In later larval stages, A. coerula larvae create sizable areas of feeding damage and completely strip leaves, leaving only the major veins ( Fig. 10b ). We conducted a simple laboratory experiment using 1-gallon potted māmaki to quantify the extent of damage A. coerula inflicts.Plants were infested with one or two newly hatched A. coerula larvae. Within two weeks, we observed 100% defoliation and the eventual death of the potted māmaki plants infested with A. coerula larvae by the second or third instar. Adults do not cause any feeding damage to the host plants.

Management. There is very little information available on how to manage this pest. In Australia, botanical insecticides (such as neem, derris powder, pyrethrum), insecticidal soaps, and microbial products such as spinosad and Bacillus thuringiensis kurstaki have been used (Jackson and Mua 2016). However, these products are not labeled for use in Hawaii on māmaki plants and are not recommended in Hawaii as they will also impact the endemic Lepidoptera populations that feed on māmaki and other endemic Urticaceae . Classical biological control is potentially the best management strategy for A. coerula .

Natural enemies. Field surveys detected egg parasitism on both Maui and Hawaii island ( Fig. 11a ). Several egg parasitoids have been found including a Trichogramma sp. ( Fig. 11b ). A gregarious, larval Eulophidae ectoparasitoid was also found on Hawaii island ( Fig. 11c ). Further work is needed to identify the parasitoids using morphological and molecular methods.

Surveying the same plants on successive trips showed more eggs than larvae present on the trees, indicating that larval predation may occur. We have not determined the species responsible but hypothesize that ants might predate on A. coerula , as they are often present on the māmaki plants surveyed. Little fire ant, Wasmannia auropunctata (Roger 1863) ( Hymenoptera : Formicidae ), was observed feeding on collected A. coerula eggs. The number of early instar larvae collected during surveys was much greater than the number of later instar larvae collected despite larger instars being more conspicuous, again suggesting that larval mortality by predation may be substantial. We frequently observed the aforementioned characteristic feeding damage on leaves during our surveys despite no larvae being found on the plant. We have also observed that the larvae are palatable to birds such as chickens ( Gallus gallus domesticus (Linnaeus 1758)) despite their bright, putatively aposematic, coloration. At least seven species of oligophagous parasitoid wasps are known to parasitize Udea stellata on māmaki (Kaufman and Wright 2010) and twelve species of parasitoids have been recorded on Vanessa tameamea ( Leeper 2014) . Further research will be undertaken to determine whether any of these resident species of wasps might provide biotic resistance to this new invasive species. If resident natural enemies do not exert significant population control on A. coerula , classical biocontrol agents may need to be sought for importation to Hawaii.

Figures

Figure 1. a. Distribution (red dots) of Arcte coerula on the island of Maui:1 Kahakuloa, 2 ʻĪao Valley, 3 Waikapū, 4 Olowalu, 5 Olinda, 6 Kēōkea; b. Distribution (red dots) of A. coerula on the island of Hawaii: 1 Honokaʻa, 2 Pāpaʻikou, 3 Hilo, 4 Mountain View, 5 Pāhoa, 6 Kalapana, 7 Volcano, 8 Pāhala.

Figure 2. a. Unhatched Arcte coerula egg (arrow), 1 mm in diameter. Laid on the underside of host plant leaves; b. Arcte coerula egg mass with 150 hatched eggs.

Figure 3. First instar larva of Arcte coerula. Black markings (arrow) on its side distinguish these larvae from other Lepidoptera that also use māmaki as a food source.

Figure 4. a. First instar larva of Vanessa tameamea; b. Larvae typically found in a tent-like protective structure at the edge of a leaf. Photo courtesy of DLNR-DOFAW.

Figure 5. a. Sixth instar larva of Udea stellata; b. Larvae typically found in a silken protective structure (arrow) on the underside of leaves with brown speckling from feeding damage.

Figure 6. Late instar larvae of tomato looper, Chrysodeixis chalcites. Although this caterpillar also has black markings on its side like A. coerula, their head capsule is green rather than tan or black and has a black line across the side (arrow).

Figure 7. a. Fifth instar yellow morph Arcte coerula larva; b. Fifth instar black morph A. coerula larva.

Figure 8. Arcte coerula pupa.

Figure 10. a. Feeding damage by 1st instar Arcte coerula larvae; b. Feeding damage by 3rd instar larvae.

Figure 11. a. Egg parasitoid of Arcte coerula emerging; b. Trichogramma sp. parasitizing the egg of Arcte coerula; c. Gregarious larval Eulophidae ectoparasitoid of Arcte coerula.

Figure9. Arcte coerula adult.The hindwingmarkings are distinctive of this species.