Eucalyptus snout beetle—Gonipterus scutellatus species complex
Eucalyptus snout beetle, also called eucalyptus weevil (family Curculionidae), as adults and larvae chews foliage of various eucalyptus species. It was a significant pest, but is now difficult to find because it is under complete biological control by a deliberately introduced egg parasitoid (parasite). What earlier was thought to be one species, G. scutellatus, is a group (complex) of eight or more look-alike species. At least G. platensis occurs in California, and the true G. scutellatus may not be present in the state.
Adults are brown and gray, about 1/3 inch long, and have an elongate, tapered head and mouthparts. When viewed from the side their body shape is domelike (convex).
Eggs are laid in a blackish to dark brown, boxlike case. Egg cases average about 1/8 inch (3 mm) long but the length varies according to the number of eggs it contains. In the case, the eggs occur side by side in groups of about 8 to 10. When the case is turned upside down the oblong eggs which are about 1/20 inch (1.2 mm) long can be seen. The eggs are yellowish when healthy and blackish when parasitized.
Larvae are white but appear yellowish green because of the sticky excretion that covers their body. Numerous short, black projections (tubercles) appear as black spots and there is a black longitudinal band along each side. Mature larvae are about 2/5 inch long. They produce frass (excrement) in strings that sometimes sticks to the larval body. The elongate pupae are yellowish, 5/16 to 7/16 inch long, and occur hidden in topsoil.
Eucalyptus snout beetles develop through 4 life stages: egg, larva, pupa, and adult. Each adult female can lay several hundred eggs. After emerging from the pupal stage, the adult weevils feed for about 1 month before laying eggs. Eggs hatch in about 1 week. The larvae develop through 4 increasingly larger instars as they chew and feed on eucalyptus foliage. Mature larvae drop to pupate in soil and during warm weather emerge as adults in about 1 month.
Adults are strong flyers and can live for several months. Overwintering is as adults clinging to eucalyptus twigs or in protected places such as under loose bark. Egg to adult development during the growing season occurs in 2 to 3 months. Eucalyptus snout beetle has about 3 generations per year.
Adults and larvae chew and feed on the foliage of various Eucalyptus species. Adults chew leaf edges giving then a notched appearance. The first two instars (larvae) scrape and chew the surface of leaves, causing pale trails in foliage. Older larvae chew entirely through leaves, leaving holes in inner portions and edges of foliage. Overall, the beetle's feeding gives foliage a ragged and scalloped appearance.
Prior to its biological control the beetle could consume almost all the leaves on large eucalyptus trees, causing distorted regrowth of shoots and death of branches and sometimes tree death. The beetle is now difficult to find in California.
Eucalyptus snout beetle is completely controlled by an egg-parasitic wasp, Anaphes nitens. The adult is a black wasp about 1/25 inch (1 mm) long that lays its eggs through the beetle’s egg case and into the eggs. A single wasp larva feeds inside each parasitized egg. The normally yellowish beetle eggs turn black when parasitized by A. nitens.
No control action is warranted for this beetle because its biological control is completely effective. Avoid applying broad-spectrum, persistent insecticides to eucalyptus trees as these can be toxic to natural enemies that help to control several eucalyptus pests.
Adapted from Identification and Molecular Phylogenetics of the Cryptic Species of the Gonipterus scutellatus Complex (Coleoptera: Curculionidae: Gonipterini) from Murdoch University of Australia, Invasion History and Management of Eucalyptus Snout Beetles in the Gonipterus scutellatus Species Complex (PDF) by the University of Pretoria in South Africa, Pest Categorisation of the Gonipterus scutellatus Species Complex (PDF) by the European Food Safety Authority, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Young larvae of eucalyptus snout beetle and their scraping chewing damage (pale lines) on a leaf.
Adult eucalyptus snout beetle (left) and chewing damage of adults and larvae.
Older larva of eucalyptus snout beetle and its chewing damage and excrement.
Adult eucalyptus snout beetle.