Mark-recapture estimates of overwintering survival of the Queensland fruit fly,Dacus tryoni, in field cages

Summary

• 1. Four groups of 75 pairs each of marked adultDacus tryoni that had emerged from field-collected guavas were liberated into cage-covered guava trees at the end of May, 1960 and sampled at weekly intervals to estimate the overwintering survivorship.

• 2. Estimates of numbers were made using three different methods as described byLeslie (1952) and one byJolly (1965). The Method A, approximate Method B andJolly models gave quite similar results although the Method A model was shown to be more sensitive to stochastic changes in theR/r ratios. The complete Method B model produced much smaller variances, due to its full utilization of the information in the data but it was very sensitive to small biases—even those caused by the restriction that recaptures must be whole numbers—and thus could not quite approach the exact expectations.

• 3. Although in some instances, statistical tests (Leslie, 1958) showed some significant deviations from randomness in recapture, these were not appreciable and did not affect the estimates.

• 4. When sufficient food was available, the flies in the field cages exhibited survival rates during the winter months of 1960 quite comparable to those of laboratory flies kept at room temperature with several exceptions: (a) there was a high mortality among the newly emerged flies during the initial week after release, (b) a high mortality during the last week of June and (c) a high mortality during the third week in July and the end of August which can be correlated with lack of rain, low humidity and warm daytime temperatures. Low temperatures apparently had little effect on survival.

• 5. There was no difference in the survival of males and females.

• 6. One replicate cage, whose flies had been anesthetized with carbon dioxide prior to release, exhibited significantly higher mortality rates than the other replicates.

• 7. Data from a fifth cage indicated that females were not inseminated during the winter months and did not produce mature eggs until the beginning of September.