Effect of tomato fruit cultivar and ripening stage on Bactrocera tryoni (Froggatt) egg and larval survival

In studies of frugivorous tephritids, determining when offspring (i.e. egg and three larval instars) mortality occurs within the fruit can greatly improve the mechanistic understanding of the fly/host interaction. Previous research has demonstrated that the Queensland fruit fly, Bactrocera tryoni, has differential offspring performance in two tomato cultivars Cherry and Roma, but when juvenile mortality was occurring was not determined. We examined B. tryoni egg and larval survival in three different ripening stages (immature-green (IG), colour-break (CB) and fully-ripe (FR)) of Cherry and Roma tomato cultivars through destructive fruit sampling at 72 and 120 hr for eggs, and 48 (1st instar), 96 (2nd instar) and 120 hr (3rd instar) after fruit inoculation with neonates for larvae. Cultivar and ripening stage had no significant effect on egg survival, nor larval survival at 48 hr: the overall percentage of egg survival was at least 80% across all treatments, while 1st-instar larval was less than 52% across all treatments. In immature-green tomatoes of both varieties, nearly all mortality occurred during the first and second instars, but at 96 and 120 hr, there were significant interaction effects between cultivar and ripening stage on larval survival. In both colour-break Cherry and Roma tomatoes, there was significant larval mortality between 96 and 120 hr. However, in fully-ripe Cherry, no further significant larval mortality happened after 48 hr, while in fully-ripe Roma significant larval mortality occurred between the first and second larval instars but not thereafter. The difference in timing of larval mortality with ripening stage provides indirect evidence of active fruit defence which is strongest in immature-green fruit, less in colour-break fruit and absent in fully-ripe fruit.