|Abstract or Summary
- A method of culturing Eriosoma pyricola on detached root pieces
was developed to ensure a regular supply of aphids for experimental
work. Another method was designed for studying the alate sexuparae
formation in woolly pear aphid, whereby growing plants were infested
so that the aphids or their progeny could be recovered later.
Sexuparae were produced in woolly pear aphid colonies on the
roots of "Domestic Bartlett" seedlings treated to induce dormancy,
indicating E. pyricola is holocyclic in Oregon. No sexuparae were
found while shoot growth continued. Shoot growth of pear seedlings
ceased when the plants were exposed to 10-hour photoperiods at a
constant 24°C. Growth continued for a much longer time when plants
were kept under 16-hour photoperiods at alternating day and night
temperatures, 24°C and 18°C, respectively. However, aphids did
not produce sexuparae when cultured on detached root pieces from plants treated for four weeks under 10-hour photoperiods at a constant
24°C. Also, sexuparae were not found in aphid colonies on root
pieces of dormant plants from the field.
All these results agree with our field observations. In the field,
sexuparae are produced for about two months in late summer and
early fall on the roots of infested plants. After this, no more sexuparae
appear in the remaining aphid populations which continue as
apterous virginoparae on pear roots until autumn of the following year.
Experiments on growth cessation in pear plants indicated that
short day-length was not the critical factor responsible for the induction
of growth cessation in pear seedlings. These experiments also
showed that constant temperatures were more important in the regulation
of growth in this pear variety.
The direct effect of photoperiods and temperatures on sexuparae
production in this aphid was also investigated. Aphids on detached
root pieces did not produce sexuparae when exposed to 10-hour photo-periods at alternating day and night temperatures, 24°C and 18°C,
respectively. Aphid cultures on detached root pieces were kept under
different combinations of 10- and 16-hour photoperiods and 15° and
18°C constant temperatures. But the treatments had no effect on sexuparae
formation. No sexuparae were found in aphid colonies on root
pieces kept in continuous darkness at constant temperatures of 15°,
18°, 21°, 24° and 27°C for periods of 14 to 49 days. Also, sexuparae were not produced when aphid cultures on root pieces were alternated
between 15° and 24°C every 12 hours for 47 days. Sexuparae were
not found in our stock cultures maintained at 21°C for about three
Direct effect of different temperatures on the development and
increase in numbers of woolly pear aphid was also studied. Apterous
E. pyricola on detached roots developed and reproduced normally at
24°C, but failed to reproduce and died within two weeks when kept at
Apterous from stock culture and alate virginoparae from elm
galls were transferred to the detached roots of "Domestic Bartlett"
pear, Amelanchier florida Lindl., Sorbus sitchensis, Crataegus
species, Ribes sativum and Red Delicious apple. The aphids did not
survive except on the pear roots.