Cold hardiness and overwintering survival of the grain aphid Sitobion avenae in Northern England

Laboratory maintained Sitobion avenae of all
developmental stages had a mean inherent supercooling
ability below -20°C. Acclimation alone, or in conjunction
with starvation had no significant effect on supercooling.
Surface moisture on the aphid cuticle during cooling
resulted in a significant loss in supercooling. Repeat
coolings to temperatures markedly above the mean
supercooling point resulted in increasing levels of
mortality.

Field collected ~ avenae showed a seasonal variation
in supercooling with a higher mean supercooling point in
the winter than in the remainder of the year. Field
populations showed a dramatic decline in December when
environmental temperatures were well above mean and
individual supercooling points. It was concluded that the
aphids were dying before they froze and that the
supercooling point was not a reliable indicator of the
lower lethal temperature.

The development and reproduction of s.avenae in the
field over winter were directly and positively related to
temperature whilst mortality increased with decreasing
temperature.

The development of a multi-channel micro-bead
thermistor unit allowed the study of a previously
unrecorded thermal event (exotherm) during the cooling of
aphids. The cause of the exotherm is unknown and warrants
further investigation. Laboratory experiments on ~ avenae
and the lupin aphid, Macrosiphum albifrons revealed that
the adults of the two species had lower lethal temperatures
(LT50) of approximately -7.5°C and -7.0°C respectively.
Nymphs of ~. ayenae were significantly more cold hardy than
adults in terms of LT50 values. Acclimation prior to
experimentation significantly improved the cold hardiness
(LT50) of s.avenae.

It is concluded that in the case of s.avenae and
other aphids supercooling points are ecologically
irrelevant if aphids are already dead before they freeze.
An experimental protocol is suggested for future
experiments on insect cold hardiness.