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common name: alligatorweed stem borer (suggested common name)
scientific name: Arcola malloi Pastrana (Insecta: Lepidoptera: Pyralidae: Phycitinae)

Introduction - Synonymy - Distribution - Description - Life Cycle - Hosts - Importance - Selected References

Introduction (Back to Top)

The alligatorweed stem borer, Arcola malloi, is a snout moth introduced to the southeastern United States in 1971 from South America as a biological control agent to suppress the growth of the water-impeding Amaranth weed species, Alternanthera philoxeroides, commonly known as alligatorweed. The alligatorweed stem borer presently occurs in Florida and other southeastern US states where it was introduced. The phytophagous larvae synergistically suppress alligatorweed, along with the alligatorweed flea beetle, Agasicles hygrophila (Coleoptera: Chrysomelidae).

The alligatorweed stem borer is related to several important pest moth species, including pests of crops, pests of stored products, and pests of turfgrass and beehives, for example the lesser cornstalk borer, flour moths, sod webworms, and wax moths. It is also a relative to the important biological control agent of invasive Opuntia, the cactus moth.

A short-winged (brachypterous) adult alligatorweed thrips, Amynothrips andersoni O'Neill.

Figure 1. The alligatorweed stem-borer moth, Arcola malloi Pastrana. Photograph by Peter Homan, University of Florida.


Synonymy (Back to Top)

Arcola malloi (Pastrana, 1961)
Vogtia malloi (Pastrana, 1961)
Macrorrhinia endonephele (Hampson)


Distribution (Back to Top)

The alligatorweed stem borer is native to South America from Argentina to Guyana (Coombs, 2004) but occurs in the southeastern United States, Australia and New Zealand where it was introduced as a biological control agent. The moth was first released in the U.S. in May of 1971 in Florida, Georgia, North Carolina, and South Carolina. It was released in Alabama in 1972 (Coulson, 1977). It was released in Australia in 1997 and in New Zealand in 1984 (CABI, 2020).

Second instar larva (deep red in color with black legs) and yellowish eggs (foreground) of the alligatorweed thrips, Amynothrips andersoni O'Neill.

Figure 2. Distribution map of the alligatorweed stem-borer, Arcola malloi Pastrana. Photograph by CABI.


Description (Back to Top)

Egg: Adult female moths deposit oval eggs singly on the undersides of apical leaves, near the leaf margin, in the midvein or in the leaf axil. Egg size is around 0.7 mm x 0.44 mm, and eggs first appear white then become yellow as the embryo develops (Center et al, 2002).

Larva: Larva are whitish and marked with wavy, tan, longitudinal stripes (Buckingham, 2004). There are 5 instars (Maddox, 1970).

Pupa: Pupa are amber colored and darken as they develop inside silken cocoons. (Buckingham, 2004).

Second instar larva (deep red in color with black legs) and yellowish eggs (foreground) of the alligatorweed thrips, Amynothrips andersoni O'Neill.

Figure 3. Larva and pupa of the alligatorweed stem-borer, Arcola malloi Pastrana. Photograph by Marylyn Feaver, University of Florida.

Adult: Arcola malloi adults are light tan, inconspicuous moths with a wingspan of up to 20 mm. Wings are dashed with brown scales on the edge and wing tip. The forewings have smooth scales with ten veins. When the moth is at rest, its wings are folded and curled partly around its body, with wing tips lying against the plant but with its head held up with the body at an angle with the plant (Buckingham, 2002).

Second instar larva (deep red in color with black legs) and yellowish eggs (foreground) of the alligatorweed thrips, Amynothrips andersoni O'Neill.

Figure 4. Arcola malloi Pastrana adults are small, tan moths that hold their wings close to the body when at rest. Photograph by Willey C. Durden, USDA ARS, bugwood.org.


Life Cycle (Back to Top)

The life cycle of the alligatorweed stem borer from egg to egg is approximately 39 days at 23 °C. Adult moths oviposit an average of 200 to 300 eggs on the underside of apical leaves of alligatorweed plants over a 6- to 8-day period. Eggs hatch in 3-4 days after oviposition. Newly hatched larvae immediately tunnel into stems through the 1st to 4th internodes of the apical stem portion and bore downwards. More developed larvae may enter stems at a lower level and burrow upwards devouring plant tissues (Center et al., 2002). As the larvae develop, they can drop down on silken threads to bore into other stems (Buckingham, 2002). Five instars of larvae develop within 24 days (Maddox, 1970). Larval feeding occurs throughout the stem. The larvae chew circular holes approximately 2 mm in diameter through the stem wall. (Center et al., 2002). The larvae seal holes with masticated tissues at the nodes along the stem to prevent water intrusion.

Pupation occurs in the stem in silken cocoons. Adult moths emerge by rupturing the thin epidermis covering the hole previously created by the mature larvae. Adults are nocturnal, remaining quiescent during the day. Females can live approximately 6 to 10 days and males approximately 5 to 9 days. The insect develops most rapidly during hot, dry periods (Kay, 1989). Only the larvae stage of Arcola malloi feed on alligatorweed (Center et al., 2002).

Three to four generations per year were reported in Argentina (Maddox, 1970), and it was reported having more than two generations annually, in the lower Mississippi River Valley (Vogt et al., 1992). It was also documented by Vogt et al. (1992) that Arcola malloi migrated to more northern regions (Arkansas and north Mississippi) from winter refuges in Louisiana at a distance up to 1000 km.


Hosts (Back to Top)

Alligatorweed (Alternanthera philoxeroides [Mart.] Griseb.) is the main host of the alligatorweed stem borer, but occasionally it will feed on Silverhead, Blutaparon vermiculare, and yellow joyweed, Alternanthera flavescens. However, development of younger larvae (1st -3rd instars) are restricted to alligatorweed (Maddox and Hennessey, 1970).

Dense stand of alligatorweed, Alternanthera philoxeroides (Mart.) Griseb.

Figure 5. Alligatorweed, Alternathera philoxeroides [Mart.] Griseb., is the main host plant of Arcola malloi Pastrana. Photograph by Shaun Winterton, Aquarium and Pond Plants of the World, Edition 3, USDA APHIS PPQ, bugwood.org.


Importance (Back to Top)

The alligatorweed stem borer, Arcola malloi, is an important natural enemy of one of the world's worst aquatic weeds, Alternathera philoxeroides or alligator weed. It was released in the United States in 1971, and since has become one of the greatest success stories in the history of biological weed control due to its ability to suppress the growth of alligatorweed along with the alligatorweed flea beetle, Agasicles hygrophila. (Kay, 1989).

Alligatorweed is an anchored, floating plant whose roots spread rapidly along waterways forming thick mats above and below the water surfaces disrupting many economic uses. Drainage canals, ditches, streams and other water bodies can become clogged with thick mats of the weed preventing emptying during heavy rains which leads to flooding, as well as causing obstructions in navigation near bridges, dams, and narrow paths in waterways (Buckingham, 2002). In addition, thick mats of alligatorweed increase mosquito populations, reduce water oxygen levels and sedimentation, and displace native plants and animals in ditches, along banks, and in shallow water (Holm et al., 1997). Alligatorweed can also infest terrestrial habitats during dry periods (Driesche et al., 2002). Alligatorweed is a noxious weed, federally prohibited in Florida, South Carolina, Arizona and California (USDA, NRCS, 1999).

Damage: The alligatorweed stem borer attacks both aquatic and terrestrial alligatorweed, but it prefers aquatic plants. It feeds within the stems of the weed causing reduced nutrient flow through thereby causing stems to collapse, turn yellow and die (Brown and Spencer, 1973).

Leaf distortion on alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. is characteristic of feeding by adults and larvae of the alligatorweed thrips, Amynothrips andersoni O'Neill.

Figure 6. Alligatorweed stems damaged internally by the larvae of Arcola malloi Pastrana wilt, turn yellow, and die. Photograph by USDA ARS, bugwood.org.

The larva is the only feeding stage of Arcola malloi. A single, developing larva can destroy up to nine stems prior to pupation (Kay, 1989). The feeding damage of the alligatorweed stem borer is synergistic with the alligatorweed flea beetle in suppressing alligatorweed when environment is optimum (Kay, 1989). The flea beetle adultsfeed on alligatorweed leaves, stripping them from the stems, while the alligator stem borer enters stems and feeds on the vascular tissues from the inside out, limiting the growth of the dense, matt-forming alligatorweed (Coombs, 2004).

Leaf distortion on alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. is characteristic of feeding by adults and larvae of the alligatorweed thrips, Amynothrips andersoni O'Neill.

Figure 7. Alligatorweed mats turn yellow but still retain leaves in contrast to the yellow stems defoliated by the alligatorweed flea beetle, Agasicles hygrophila Selman and Vogt. Photograph by USDA ARS, bugwood.org.


Selected References (Back to Top)

Authors: Bonnie C. Wells and Carey Minteer, University of Florida
Photographs: Peter Homan, University of Florida, CABI, Marylyn Reaver, University of Florida, Willey C. Durden, USDA ARS, bugwood.org., Shaun Winterton, USDA APHIS PPQ, Bugwood.org., USDA ARS, bugwood.org.
Web Design: Kay Weigel
Publication Number: EENY-764
Publication Date: August 2020.

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