Full-text - Norsk entomologisk forening

Overwintering Pelophila borealis. 111. 133
Table 1. Mortality at -lo0, -5O, and -3' C of
frozen adults of P. borealis previously acclimated
at O0 C
-
Temp. Storage Percent
c time n mortality
-lo0 10 min. 5
1 hr 19
- 5O 4 days 6
8 » 13
12 . 5
16 * 24
32 n 12
48 19
- 3" 16 days 7
32 n 12
64 n 8
Table 111. Mortality at -5' C of frozen adults of
P. borealis collected at various times of the year
Date of Time at Percent
collection -5O C n mortality
4 Sept. 1972 1 hr 5 100
2 hrs 8 1 00
23 Oct. 1972 4 days 4 O
8 n 8 38
20 D 7 1 O0
17 Dec. 1972 8 days 1 O
16 )> 1 50
27 Jan. 1973 8 days 6 O
24 >* 12 75
23 March 1973 12 days 4 25
20 8 63
26 D 3 66
Table 11. Mortality at -5O C of frozen adults of
P. borealis previously acclimated at -5" C for
two to three weeks
Time
Percent
at -5' n mortality
8 days 7 14.2
12 >> 4 o
16 >> 11 18.2
32 )> 12 33.3
48 n 13 69.2
were dead after 32 days, and 25 per cent
only were dead or moribund after 64 days at
this temperature.
Table 11 presents the mortality of beetles
acclimated in a supercooled state for 2 to 3
weeks at -5'C before being frozen at this
temperature. The beetles were removed from
their overwintering sites at the same time as
those used in the experiments described above.
Compared to beetles kept at O°C (Table 1),
higher survival rates were found in beetles
acclimated at -5°C. In the last group more
than 65 percent survived 32 days in frozen
condition at -5' C, and about 30 percent survived
48 days.
Finally, some experiments were performed
to see if the freezing tolerance of the beetles
changed with the time of the year. The
results are presented in Table 111. Large differences
were found between beetles collected
in early and late autumn. Those collected in
September were killed by short exposures at
-5OC, while those collected in the end of
October showed relatively high survival rates.
Although the results are based on a small
number of beetles, the degree of freezing
tolerance appeared to be still higher in
beetles removed from their overwintering
sites in December, January and March.
DISCUSSION AND CONCLUSIONS
From the present study it is concluded that
adults of P. borealis can survive freezing at
least at temperatures down to -5"C, while
al1 beetles die at -lO°C. The survival rates
are not only dependent on temperature, but
also on the time spent in frozen condition.
Thus most of the beetles were dead after
one month at -5OC, while most of them survived
two months at -3OC. The degree of
freezing tolerance changes with the time of
the year, and may also increase if the beetles
are exposed to sub-zero temperatures in a
supercooled state.
In the spring, survival rates of about 90
percent have been found in beetles that have
overwintered in their natural habitats (Conradi-Larsen
& Ssmme 1973). The low mortality
may partly be explained by the ability of
the beetles to supercool (0stbye & Somme
1972), and partly by their ability to survive
freezing. While lowest ground temperatures
are recorded in the autumn before a thick
snow cover is accumulated, the soil remains
relatively dry. Under these conditions the
beetles probably remain unfrozen, thus
avoiding the harmful effects of freezing at
low temperatures. Even if freezing occurs,
the beetles have a fair chance of survival, at
least at temperatures down to about -5'C.
9 - Norsk ent. Tidsskr.