Fly emergence generally begins about the end of June or beginning of July, reaching a peak by the beginning of August, and virtually ends by late August or early September. Also, about 5 percent of the flies carry over into the second summer before emerging. This carryover enables the fly to survive a year of total crop failure. A rule of thumb to remember is that emergence begins about 30 days after McIntosh petal fall. Accumulated heat units from early May until emergence time and soil texture are probably the most important factors influencing time of fly emergence.
Flies emerge earlier from light sandy soils than from heavy or wet soils.
Egg laying does not begin immediately after fly emergence. Instead, the flies rest and feed on the foliage of apples and other trees and shrubs either in or near the orchard. Mating occurs at this time. In general, egg laying begins about 10 days after emergence. Each apple variety, whether early or late, has a rather definite period of attractiveness to the flies that probably coincides with the degree of fruit maturity.