544 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
of development will be present in the field at almost any time. In 
some species, however, not all of the adults issue from the cocoons in 
the same season, but some emergence may be delayed until later in 
the year, or there may be a diapause of one or more years. This 
division of the period of adult emergence must occasionally make 
possible the continuance of an infestation which would otherwise 
cease on account of temporary adversities. 
Although reproduction is largely bisexual, studies in recent years 
have shown that there are some exceptions. Some examples may be 
cited, such as Diprion hercyniae (Htg.), in which the males are ex- 
tremely rare; Phyllotoma nemorata Fall., of which no males are known 
to exist; and D. simile (Htg.), in which parthenogenetic reproduction 
has been obtained experimentally, but the progeny on reaching the 
adult stage were all males. 
The relationship between sawflies and their host plants seems to be 
a somewhat limiting one, and apparently few species are able to sub- 
sist on a wide range of plants. As in many other species of insects, 
local outbreaks may occur at irregular intervals, although few species 
of sawflies have been reported as exceptionally important pests. In 
recent years, however, more species have been attracting attention as 
pests in our forests and plantations. Some of these are of foreign 
origin and of comparatively recent establishment in the United States. 
The large-scale planting of pure stands of forest trees, particularly 
conifers, undoubtedly has also favored the increase of some native 
species of sawflies. 
The fluctuation of sawfly populations is influenced by many factors, 
such as climate, disease, natural insect enemies, and rodents and other 
predators. These factors have not been extensively studied and are 
therefore not too well understood. Control by direct measures is 
difficult and in many cases impractical under forest conditions. Where 
the growth is of sufficient value to warrant the expense, effective con- 
trol of sawflies can usually be obtained by a timely spraying of their 
food plants with a stomach poison for the external feeders, or an 
ovicide, such as nicotine sulfate, for the leaf miners (p. 53). 
Some of the more important families and species are discussed in the 
following pages. Keys, which are not intended to be broadly inclu- 
sive, are given to the larvae of species most frequently encountered in 
the woodlands or in forest plantations. Although it may be found 
that some insect descriptions and insect injuries will not fit the require- 
ments of these keys and that some will run to a place in the keys where 
they do not belong, this situation cannot very well be remedied in 
those keys which do not include all the species of any group and in 
which many technical characters are avoided for the sake ot simplicity 
and usefulness. 
The keys, unless otherwise stated, apply only to larvae in their last 
feeding instar. 
KEY TO THE LARVAE OF SOME OF THE MORE IMPORTANT SPECIES OF SAW- 
FLIES AND HORNTAILS THAT ATTACK TREES AND SHRUBS 
iF Sawfly larvae feeding on coniferous trees, or the Pinaceae_______- 2 
Sawfly larvae feeding on broad-leaved trees or shrubs___________- 9 
2. Larvae solitary or gregarious, but not in nests of webbing and frass_ 3 
Larvae in nests of webbing and aso ee ee 8 
3 Food plants, species of the Pinaceae other than pine___-_-_------- 4 
Food: plants,“pines 2 Lt ieee Sa ee re Se ee 5 
