High cold tolerance and differential population response of third instars from the Zeugodacus tau complex to phytosanitary cold treatment in navel oranges

The scarcity of phytosanitary treatment options for polyphagous pests of the Zeugodacus tau complex poses a quarantine risk for importing countries. Owing to the unresolved taxonomy and a limited number of cold treatment schedules available for this group, we tested whether populations of the Z. tau complex infesting Citrus sinensis differ in their cold tolerance and determined the efficacy of a single cold treatment against the apparently most tolerant of four populations tested. Additionally, the susceptibility and suitability of C. sinensis to Z. tau populations were assessed in field cages. Treatment efficacy was determined by larval mortality. Tests comparing the tolerance of Z. tau third instars from wild strains of Palampur (India), Fujian (China), and Baipayl (Bangladesh) and a laboratory strain from Fujian to cold treatments (≤ 1.70 ºC for 3, 8, 10, 13, 15,16, 18, and 20 days) showed that Fujian-wild and Palampur were the most cold-tolerant populations, although the differences at > 99.9% mortality, the level that treatments must achieve, were not significant at the 95% confidence level. Confirmatory tests exposing > 30,000 third instars from Fujian-wild and Palampur to average temperatures ranging from 1.19 to 1.45 ºC for 22 days yielded four survivors, however, none of the four were able to develop to the adult stage. Despite the low levels of infestation observed in C. sinensis, Z. tau could infest intact harvested oranges placed on trees under field cage conditions and sustain its development to the adult stage. Even though acute larval mortality has been considered the endpoint of cold treatments for tephritids, the low susceptibility and suitability of oranges combined with the cold-treated survivors’ inability to develop into adults may be relevant factors for evaluating whether 22 days exposure to ≤ 1.70 ºC provides quarantine security for Z. tau. This study provides some confidence that future taxonomic divisions that may result within the Z. tau complex might be controlled by generic cold treatments that would have the same time duration at a given temperature.