Tree Fruit and Grape Industry News, March 1999

Results of 1998 Tentiform Leafminer Survey

The growing season of 1998 started early and experienced record high summer temperatures. These conditions contributed to the development of serious tentiform leafminer infestations in several apple orchards throughout the apple-growing areas of the southern Interior. The primary parasite, Pnigalio flavipes, was unable to keep up with the pest during the first and second generations of leafminers. However, the third generation of leafminer appearing as the summer wound down was not so lucky.

In October, a survey of apple orchards was conducted to assess the proportion of tentiform leafminer larvae killed by adult and larval parasites feeding on the larvae. One hundred leaves were collected from five randomly selected orchards near Vernon, Winfield, Kelowna, Summerland, Naramata, Keremeos, Cawston, Oliver and Osoyoos; one orchard was sampled near Creston. Up to 100 mines were examined from each sample and the number of live and dead life stages of leafminers and parasites was recorded.

The results of this survey indicate that, once again, the parasite(s) exacted a heavy toll on the leafminer. Overall mortality of leafminer larvae due to parasite attack was greater than 96 per cent. These results are similar to those recorded in a survey conducted in the fall of 1992 after a season of high leafminer abundance. No live leafminer pupae were found in this recent survey, and this is the stage in which the leafminer is best able to pass the winter. Very few live sap-feeder or tissue-feeder larvae were found compared to the number of parasite pupae (the overwintering stage). The milder winter this year may allow some of the tissue-feeders to survive, and will also improve the overwintering success of the parasites.

Therefore leafminers will be very scarce this summer with the possible exception of orchards that did not have many in 1998. These latter orchards may have high numbers of leafminers only if the growing season duplicates that of 1998. 

For more information contact:
Hugh Philip, Entomologist, 
BCMAFF, Kelowna

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