Emergence of blackcurrant gall mite (Cecidophyopsis ribis) from galls in spring

The emergence of the blackcurrant gall mite, Cecidophyopsis ribis (Westwood ), from galls on the blackcurrant cultivars Ben Lomond and Ben Tirran was m onitored closely in 1995-1999 using miniature sticky traps. Emergence was p receded by swelling of the galls. First, 5% and 50% emergences varied from Julian day 74-112, 84-121 and 101-129 respectively in the different years b ut were virtually identical on the two cultivars, even though 'Ben Tirran' flowered on average 14 d later than 'Ben Lomond.' First emergence was often associated with the first day after 1 March with a maximum air temperature greater than or equal to 16 degreesC. More satisfactory predictions of the seasonal timing of emergence were made by accumulated temperature sums abo ve a threshold of 4 degreesC from Julian day 46 (15 February). The average accumulated temperatures for first, 5% and 50% emergences were 122, 199 and 316 degree-days which gave mean errors in the predictions of 3.1, 1.3 and 7.2 d respectively. In 1998, the onset of emergence was delayed by a prolon ged period of wet weather during which the internal tissue of the galls bec ame necrotic, particularly on 'Ben Tirran.' The accumulated temperature mod el predicted the first, 5% and 50% emergences to within 4, 4 and 5 d respec tively in 2000. There was great day-to-day variability in the mean number o f mites that emerged. There was a positive correlation between the number o f mites emerging and mean or maximum daily temperature but numbers were sup pressed on days with rainfall. There was great variability in the timing of emergence of mites from individual galls and the variance of the number of mites emerging was related to the mean according to Taylor's power law. Th e emergence of mites had a strong diurnal rhythm, controlled by both light and temperature, with virtually no mites emerging between 2300 hours and 09 30 hours. Galls contained several thousand motile mites and roughly double the number of eggs during the early part of the migration when the internal tissue of the galls was green and succulent. The internal tissue of the ga lls became progressively dry and chlorotic as the migration progressed and the numbers of mites and the emergence dwindled and eventually ceased. Impl ications for the control of the gall mite with acaricides are discussed.