Biology Reference
In-Depth Information
Another interesting variation of larval diapause is the occurrence of sta-
tionarymolts during the diapause stage. For example, larvae of the diapausing
noctuid
Sesamia nonagriodes
feed slowly and undergo up to 12 (although usu-
ally 3-4) stationary larval molts without increasing the body size. In such lar-
vae, ECD rises irregularly during the extra larval molts but high JH levels
prevent pupation. Topical applications of an ECDmimic to diapausing larvae
induce them to accelerate larval molts rather than induce pupation and non-
diapausing fifth instar larvae make extra larval molts when fed with a JH ana-
logue resembling the induction of diapause. Normally, diapause is only
specified prior to the third instar and results in an increasing JH titer that keeps
them in a juvenile form (
Eizaguirre et al., 1998; Eizaguirre, Schafellner,
Lopez, & Sehnal, 2005
). Similarly, high levels of JH maintain larval diapause
in the yellow-spotted longicorn beetle,
Psacothea hilaris
(
Munyiri & Ishikawa,
2004
). Many Lepidoptera also exhibit stationary molts during diapause
(
Kevan, 1944; Sugiki & Masaki, 1972; Usua, 1973; Yagi & Fukaya, 1974
).
While stationary molts are relatively common in many species, some just
arrest with no extra molting. In such cases, diapause is thought not to be
induced by prolonged JH pulses, but rather, by a failure of ECD signaling
(
Denlinger et al., 2012; Saunders et al., 2002
). For example, injection of
JH into nondiapausing
Ostrinia nubilalis
larvae fails to induce dormancy,
but if ECD is injected into diapause-induced larvae, they will undergo a lar-
val to pupal molt instead of a stationary larval molt (
Denlinger et al., 2012;
Gadenne, Varjas, & Mauchamp, 1990; Gelman & Brents, 1984; Gelman
et al., 1992; Gelman & Woods, 1983; Peypelut, Beydon, & Lavenseau,
1990
). In larvae of both
Laspeyresia polmonella
and
Ostrinia nubilalis
the loss
of ECD signaling appears to be a result from a block in the brain-derived
prothoracicotropic hormone (PTTH) signaling cascade that is necessary
for high level ECD induction in most insects (
Bean & Beck, 1980, 1983;
Denlinger et al., 2012; Saunders et al., 2002; Sieber and Benz, 1977,
1980; Smith & Rybczynski, 2012
).
5. PUPAL DIAPAUSE
The final developmental stage in which diapause can be induced occurs
just after the larval to pupal molt, but before significant metamorphosis occurs.
As described above for some larval arrest examples, pupal diapause comes
about through an inhibition of ECD synthesis (reviewed in
Denlinger
et al., 2012; Saunders et al., 2002
). These arresting pupae can be extremely
cold tolerant (
Lee & Denlinger, 1991
) as described previously for hatch-ready
