- Food habits and biology of five species of Pleocoma larvae were
studied at a dozen forested sites in western Oregon between May 1960
and December 1961.
First instar Pleocoma hatch in late summer and moult to second
instars in early fall. Second and subsequent instars moult annually
between mid-summer and early fall. Larvae appear to go through
more than nine instars pupating after the seventh, in the upper 20
inches of soil, in mid-summer. Male larvae outnumber females by
about 30 percent.
Larvae move through the soil primarily by use of the mandibles.
This movement can exceed a rate of four inches a day.
Larval populations varied from none to 4.4 larvae per square
foot and were distributed between two and 44 inches in depth. Soil
temperatures and soil moisture influenced most larvae at some sites
to leave the upper 16 inches of soil during the summer. At other
sites, however, a shallow silicate clay hardpan influenced larvae to
remain at shallow depths throughout the year. A fungus disease
killed from five to more than 20 percent of the larvae in some areas.
Dipterous predators killed some larvae.
Coniferous roots comprise the major part of the larval diet
throughout most of the year, being found exclusively in 86 percent
of the larvae. Thirty percent of the roots in the guts were definitely
identified as Douglas-fir, the predominant conifer at the sites of
the collections. Larvae preferred smaller roots, mostly smaller
than 2mm, many of them mycorrhizal rootlets. A few larvae merely
girdled the roots, stripping the bark leaving the xylem in the soil.
Most larvae, however, severed and consumed the entire root.
Larvae either severed and ingested intact root segments or gnawed
on root ends masticating them into very fine pieces before ingestion.
Except for last instar larvae, usually at shallower depths, and
first stage larvae, neither of which appeared to feed on roots, all
other larval stages at all depths were feeding on roots throughout
the year, except during the moulting period. The cessation of root
feeding extended over about a four-month moulting period from
June to September, inclusive. The moulting period varied somewhat
between species. Most larvae consumed the exuvia. Some soil,
probably less than five percent by volume, was ingested.
Other material tentatively identified in larval guts was: remains
of fungal hyphae, cast ventricular epithelium, gregarine parasites, and bacteria.
Pleocoma larvae do not appear to be serious forest pests in old-growth and advanced second-growth coniferous forests because of the
generally low and scattered larval populations. In newly-established
forests, however, Pleocoma larvae are a potential pest as one or
two feeding larvae can kill a small tree. Only further studies in regenerated
areas will determine their role in these cut-over and reestablished