pdf, 57.71Mb - Entomological Society of Canada

132 P. Harris and J. H. Myers
Metzneria
paucipuncteJJa
Zell. (Lepidoptera:
Gelechiidae)
Table 29
The life cycle of the moth on C. vaJlesiaca in the Swiss Rhone Valley (the source of the
stock released in British Columbia) is reported by Englert (1971, 1972). The moth is
univoltine and emerges at the end of May. It lays 60-100 eggs with a maximum ofthree eggs
on an individual flower head base or adjacent stem. A single larva survives to develop in a
head where it feeds principally on the achene while the seed coat is still soft. The mature
larva overwinters in the seed head which in spotted knapweed remains standing all winter
having shed its seed in the fall. Up to nine seeds are eaten by a larva and in 83 attacked
heads of C. vaJlesiaca, 95% of the viable seed was destroyed. Usually one-third to one-half
of the heads were attacked even though the plants were widely scattered. Approximately
20% of the eggs and 30-40% of the larvae were parasitized in Europe. Field records of the
moth are restricted to knapweeds in the section MacuIosae, although in the laboratory it
developed on C. diffusa.
Adult M. paucipunctella were released (Table 32) in 1973 and 1974 at Castelgar airport
and Westwold, British Columbia. M. paucipunctella has not been recovered from
Castelgar; the number released was slightly less than at Westwold (73 in 1973 and 76 in
1974) and in 1974, the flowering plants were mown shortly afterthe release was made. At
Westwold, the initial establishment was tenuous with only four larvae found at the
release point in August 1973. By the following year, larvae were found up to 20 m away
(Table 29) and in 1976, they had spread a radius of at least 50 m. By 1979, the population
was evenly distributed within a 50 m radius and by 1981, M. paucipunctella had spread at
least 0.5 km.
Establishment of M. paucipunctella Zell. on spotted knapweed, C. maculosa Lam., at
Westwold, British Columbia.
Year
(fall)
1973
1974
1976
1977
1978
1979
1980
Radius Sampled
(m)
25
20
50
30
25
50
15
No. Heads
Examined
4351
2999
485
1443
1719
531
443
% heads with a M.
paucipunctella larva
0.1
0.7
2.0
3.7
27.5
20.3
37.7
The population growth of M. paucipunctella has reflected the winter mortality of
larvae in the previous year. For example 68% of the larvae survived the winter of
1977-78 compared with only 28% in 1978-79. The reason for the mortality is not clear.
The larvae can survive colder temperatures than they experience at Westwold; survival in
the mild humid winter of Vancouver in 1977-78 was 82%, and it was 78% in the dry cold
winter of Walachin, British Columbia. Possibly larvae are affected by a combination of
moisture and cold and the rate of onset of cold weather. Larvae absorb and lose moisture
readily: 14 overwintering larvae dissected from knapweed heads and put on moist filter
paper at 1000C in 3 days increased their weight by 50% (from 3.17±0.56 g to 4. 79± 1.13 g).
After 16 days in a desiccator, they had returned to their original weight of 3.11±0.66 g.
Less than 1% of the larvae are parasitized by an Elachertus sp. (Chalcidae) and some
larvae are eaten by mice which feed on the seed heads particularly in the late summer.
The percentage of seed destroyed by M. paucipunctella in Canada is much lower than
the 95% reported by Englert (1971). However, C. biebersteinii heads at Westwold