Acantholyda adults are about 10–17 mm in length with long thread-like antennae. There are two subgenera that can be distinguished by a ridge behind the eye. They have distinctive larval habits that involve building elaborate nests of webs, leaf material, and frass, inside of which they feed and keep safe from predators. Some species are significant pests of conifer trees (Middlekauff 1958, Shinohara and Byun 1996).
There are 78 described species worldwide. Thirty-seven species occur in North America, including one introduced from Europe (Taeger et al. 2010).
A Key to North American species of Acantholyda is included in Middlekauff 1958.
Pamphiliidae are recognized by a somewhat quadrate head and tarsal claws with inner teeth. Acantholyda can be distinguished from Cephalcia by the preapical spur on the inner side of fore leg and from other genera in the family by the small inner tarsal claw tooth (Goulet 1992).
Acantholyda nipponica, a closely related species to A. erythrocephala, is a pest of Larix leptolepis (Japanese larch) in Japan. This species also feeds on several Pinus sp. (pine). Acantholyda sasakii was recorded as a pine pest in in the early 1900s, with Pinus densiflora (Japanese red pine) a known host plant (Shinohara 2001).
Acantholyda posticalis is a significant defoliator of pines in Europe and Central Asia, most notably of Pinus sylvestris (Scots pine). Outbreaks have been reported in Poland, Germany, Russia, and Kazakhstan. In Lithuania during the 1970s, an outbreak damaged over 14,300 acres of pine forest (Voolma et al. 2009). Northern populations of this species have a documented cold tolerance of more than -20°C, and it is being monitored for range expansion to the north (Voolma et al. 2016). This adaptability may increase the chances of successful establishment in North America.
Larvae feed on needles of numerous species of Pinaceae. Host species of pine include: Pinus taeda (loblolly pine), Pinus contorta (lodgepole pine), Pinus ponderosa (ponderosa pine), Pinus muricata (bishop pine), Pinus resinosa (red pine), P. banksiana (jack pine), P. strobus (eastern white pine), Pinus rigida (pitch pine), Pinus mugo (mountain pine), Pinus sylvestris (Scots pine), Pinus densiflora (Japanese red pine), Pinus nigra (black pine), Pinus radiata (Monterey pine), Pinus sabiniana (foothill pine), Pinus edulis (Colorado pinyon), Pinus elliottii (slash pine), Pinus clausa (sand pine), Pinus monticola (western white pine), and Pinus pungens (table mountain pine). Other hosts are trees of Tsuga heterophylla (western hemlock), Pseudotsuga menziesii (Douglas fir), Picea glauca (white spruce), Picea sitchensis (Sitka spruce), Picea mariana (black spruce), and other Picea sp. (Middlekauff 1958, Greenbaum 1975).
The female makes a small slit in the side of a mature needle and deposits eggs so that the membrane gets caught in the slit, ensuring the eggs stay in place on the outside of the needle, yet allowing the eggs to be in contact with vascular tissue to obtain moisture. After hatching, the larvae commence building a structure out of silk and plant material for protection, within which they safely feed. Some species feed gregariously. They eat by using mandibles to cut small needles at the base, then bringing the needle to the shelter, and devouring it from base to apex. Mature larvae measure up to 25 mm in length, have reduced prolegs and are variable in color (Middlekauff 1958).
When mature, larvae fall to the ground and burrow into the soil to a depth of about 5–8 cm. The prepupa overwinters in a small cell built from soil and silk. In spring, it pupates and emerges. Adults generally fly in spring, during April and May. Many Acantholyda are univoltine (Middlekauff 1958).
Acantholyda erythrocephala, commonly known as the pine false webworm, is a significant pest of pine trees and has historically caused severe defoliation in North America, Europe, and Asia (Middlekauff 1958). For example, 570,000 acres in New York was heavily infested between 1989 and 1999 (Asaro and Allen 2001). Damage sustained by trees from several years of feeding by A. erythrocephala can provide habitat for secondary pests such as bark beetles, and can result in mortality. Christmas tree plantations often sustain significant economic losses due to the presence of this pest (Kenis and Kloosterman 1999).
World: Outside of the Americas, the range of Acantholyda is restricted to the Northern Hemisphere, with species known throughout Europe, and in Asia east to Korea and Japan (Shinohara and Byun 1996, Taeger et al. 2010).
North America: Acantholyda are common in forested regions of northern North America (Middlekauff 1958). Four species are known to occur in Mexico, as far south as Veracruz (Smith 1988). Acantholyda erythrocephala, an introduced European species, was first discovered in Pennsylvania in 1925. It spread east to New Jersey, New York, and New England, then west to Wisconsin, Ontario, and Alberta (Middlekauff 1958, Asaro and Allen 2001).
Map data from: GBIF.org (26 June 2019) GBIF Occurrence Download Acantholyda
Details about data used for maps can be found here.