Year-round collection of willow sieve-tube exudate.

Seasonal variations in sieve-tube sap were followed by collecting exudate from the broken stylets of an armored scale insect (Quadraspidiotus ostreaformis Curtis) which overwintered on 3- to 6-year-old branches of a local willow population (Salix exigua Nutt.). During the winter, the scale insects fed on functional sieve tubes near the cambium. Broken stylets on stem segments taken to the laboratory exuded promptly even when collections were made at temperatures as low as-30°C. Exudation in the field was observed at-11 to-13°C. Sucrose accounted for virtually all of the sugars in exudate collected during the winter. Sieve-tube sap collected during the winter was about twice as concentrated as summer sievetube sap. The concentration decreased during rapid vegetative growth in the spring, and increased again as growth slowed in late summer and early fall.