Ds. Koveos et A. Veerman, ACCUMULATION OF PHOTOPERIODIC INFORMATION DURING DIAPAUSE DEVELOPMENTIN THE SPIDER-MITE TETRANYCHUS-URTICAE, Journal of insect physiology, 40(8), 1994, pp. 701-707
Photoperiodic control of diapause development was studied in three str
ains of the spider mite, Tetranychus urticae, originating from differe
nt latitudes in Europe. Diapause was almost fully maintained by a long
-night regime during the first month of diapause, but was terminated r
apidly and synchronously by a short-night regime. Diapause termination
in continuous light was almost as rapid as in short nights; in contin
uous darkness diapause termination proceeded much slower, probably rev
ealing the ''spontaneous'' rate of diapause development of these mites
. The effect on diapause termination of successive short-night cycles
appeared to be accumulated in a way comparable to cycle summation duri
ng diapause induction. The threshold for diapause termination by short
nights, expressed as the number of cycles required for 50% diapause t
ermination, was found to be lower the more southern the origin of the
strain of mites. However, considerable differences in the number of cy
cles required for diapause termination were also present within strain
s: some mites needed only 3-4 short nights to terminate diapause, wher
eas others needed more than 10. In contrast with short-night cycles th
e effect of long-night cycles was not accumulative. Intensification of
diapause under the influence of long-night cycles during its early st
ages appeared not to take place. Long nights were capable of maintaini
ng the state of diapause if experienced before short-night cycles, but
not afterwards. Short and long nights seem to act independently from
each other, the former accelerating diapause development (''activation
''), the latter slowing down diapause development (''diapause maintena
nce''). No antagonistic effect of long and short nights was found on t
he photoperiodic maintenance of diapause, in the sense that the effect
of short nights might be diminished or even reversed by long nights,
and vice versa, as found in photoperiodic induction of diapause in T.
urticae. Therefore diapause maintenance does not seem to involve a pho
toperiodic counter mechanism comparable with the counter involved in t
he photoperiodic induction of diapause in these mites.