Cold Tolerance of the Mediterranean Fruit Fly in Date and Mandarin

The applicability of species sensitivity distributions to the development of generic doses for phytosanitary irradiation

Ionizing radiation is used as a phytosanitary treatment to prevent the introduction of pests through trade. Generic doses are a valuable means to increase the number of pest-commodity combinations that can be treated using phytosanitary irradiation. Generic doses allow for the treatment of the entire taxa for which the dose has been approved, allowing for the treatment of untested species. As such, the approval of a generic dose requires substantial supporting data and careful consideration of the risks involved. We adopt the Species Sensitivity Distribution (SSD) framework, already in widespread use in the field of ecotoxicology and environmental risk assessment, to evaluate generic doses for phytosanitary irradiation treatments. Parametric SSDs for Curculionidae and Tephritidae were developed using existing data on efficacious phytosanitary irradiation treatments. The resulting SSDs provided estimates of the taxa coverage expected by the generic dose, along with the margin of uncertainty. The SSD analysis lends support to the existing 150 Gy generic dose for Tephritidae and a proposed 175 Gy generic dose for Curculionidae. The quantitative estimates of risk produced by the SSD approach can be a valuable tool for phytosanitary rule making, improving the process for generic dose development and approval.

Relative Tolerance of Three Morphotypes of the Anastrepha fraterculus Complex (Diptera: Tephritidae) to Cold Phytosanitary Treatment

The Anastrepha fraterculus (Wiedemann) complex is currently comprised of at least eight morphotypes, including several that are likely to be described as new species. It is critical to evaluate whether the morphotypes differ in tolerance to phytosanitary treatments. Temperatures from 0 to 3°C are used as a phytosanitary treatment for some commodities exported from the region and at risk of infestation by the A. fraterculus complex. Description of A. fraterculus morphotypes as new species could result in the annulation of phytosanitary treatment schedules for the new species. This study compared the relative cold tolerance of five populations from three morphotypes of the A. fraterculus complex: Andean, Peruvian, and Brazilian-1. Both a laboratory and wild strain of the Brazilian-1 morphotype were studied. Differences in mortality of third instars of the five A. fraterculus populations reared on nectarines were observed only with short treatment durations at temperatures ranging from 1.38 ± 0.04°C to 1.51 ± 0.08°C (mean ± SEM). Estimated times to achieve the LT99.99682 (probit 9) showed that Brazilian-1 wild, Brazilian-1 laboratory, and Cusco population were the most cold tolerant, followed by Andean and Peruvian, the least cold tolerant morphotype (i.e., Brazilian-1 wild = Brazilian-1 laboratory = Cusco population > Andean > Peruvian). These findings suggest that the current cold treatment schedules of 15 d at ≤ 1.11°C and 17 d at ≤ 1.67°C can be applied as cold treatments to any potential new species that may arise from the A. fraterculus complex.

Comparison of Populations of Ceratitis capitata (Diptera: Tephritidae) from Three Continents for Susceptibility to Cold Phytosanitary Treatment and Implications for Generic Cold Treatments

The Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), is arguably the most significant and studied quarantine pest of fresh fruits. There is well over a century of research observations on its response to cold, first as it pertains to shipment of fruits using cold temperatures to preserve fruit quality and how that may aid the survival and distribution of the pest, and then the use of colder temperatures to kill the pest in fruit shipments. Cold tolerance at 1.1°C in three populations of C. capitata generally increased as the insect developed; therefore, the third instar is the most tolerant of the stages that are found in fruit. The three populations did not differ in cold tolerance, indicating that cold phytosanitary treatments against this pest can be harmonized regardless of country of origin of marketed fruit hosts. This study facilitated the approval of some cold treatment schedules for the International Plant Protection Convention treatment manual that were being held up by concerns of possible differences in cold tolerance among C. capitata populations from different countries and points toward the possibility of generic, broadly applicable phytosanitary cold treatments. Most larvae found alive after 9 d of cold treatment did not pupariate and fewer still emerged as adults, indicating that acute larval mortality need not always be the objective of a cold phytosanitary treatment to be efficacious in preventing the establishment of invasive species.

Cold Response of the Mediterranean Fruit Fly (Ceratitis capitata) on a Lab Diet

Cold treatment at 0.0 °C with different exposure durations (0–12 days) was applied to the Mediterranean fruit fly Ceratitis capitata (Wiedemann) fed on a lab diet. The examined developmental stages were early eggs (<6 h), late eggs (>42 h), first instar, second instar and third instar larvae. Pupation, adult emergence and sex ratios of survived flies were investigated to study the C. capitata responses to this low temperature treatment. Our results showed that exposure time at low temperature has a clear effect on pupation and adult emergence. Based on pupation ratios, the first and third instar are the most cold tolerant stages, with LT99 = 7.3 for both of them. Cold tolerance at both stages are very close and no significant differences were detected. There were no significant differences on C. capitata sex ratios among different stages after treatment. This study improves our understanding of C. capitata responses to cold treatment, which may assist in the improvement of the current treatment strategies to control this destructive horticulture pest species.

Comparative Cold Tolerance in Ceratitis capitata and Zeugodacus cucurbitae (Diptera: Tephritidae)

Cold tolerance studies were conducted with the egg and larval stages of Mediterranean fruit fly, Ceratitis capitata (Wiedemann) and melon fly, Zeugodacus (Bactrocera) cucurbitae (Coquillett) in Navel oranges to determine whether quarantine cold treatments approved for C. capitata might also be effective against Z. cucurbitae. Navel orange is a good host for C. capitata and a poor host for Z. cucurbitae, and therefore, artificial infestation of fruit was used to facilitate comparisons. Laboratory-reared eggs and larvae were inserted in the center of Navel oranges, placed in cold storage chambers at 1.5-2.0°C for 1, 2, 4, 6, 8, 10, 12, or 14 d, then removed, and evaluated for egg hatch or larval survival. Time-response data were analyzed using linear mixed-effects models, probit analysis, and visual inspection of survivorship graphs. C. capitata eggs were significantly more cold tolerant than Z. cucurbitae eggs, and Z. cucurbitae larvae were generally more cold tolerant than C. capitata larvae. C. capitata eggs and Z. cucurbitae second instar larvae were the most cold-tolerant life stages, and they were not significantly different from each other. Results suggest that cold treatment at ≤1.5°C for a minimum of 14 d would be sufficient to achieve disinfestation of C. capitata and Z. cucurbitae. The inherent cold tolerance in Z. cucurbitae is equal to or higher than that of C. capitata, and therefore, cold treatment protocols developed for C. capitata may not always be effective against Z. cucurbitae.

An Additional Phytosanitary Cold Treatment Against Ceratitis capitata (Diptera: Tephritidae) in 'Oroblanco' Citrus Fruit

For 'Oroblanco' ('Sweetie'), the sweet seedless pummelo-grapefruit hybrid, when exported from Israel to Japan, the standard cold treatment against Ceratitis capitata (Wied.) (Diptera: Tephritidae) is conducted at ≤ 1.5 °C, for 16 d. In recent years, the transportation means of exported citrus was changed from reefer vessels to individual refrigerated containers, where the fruit bulk is relatively small and may be exposed to temperature fluctuations and to the risk of chilling injuries. To reduce this risk, Israel proposed to Japan to increase the treatment temperature and extend its duration to 2.2 °C and 18 d, respectively. This study shows that the proposed treatment effectively kills the third instar larva of C. capitata, in Oroblanco.

Supercooling Capacity and Cold Tolerance of the Wild Silkworm, Antheraea pernyi (Lepidoptera: Saturniidae)

While wild silkworms have served humans for several thousand years, little attention on cold hardiness has been paid to these economically important species. In the present study, supercooling capacity and low temperature tolerance of Chinese oak silkworm, Antheraea pernyi (Guérin-Méneville) (Lepidoptera: Saturniidae), an economic insect reared both for silk production as well as human food, were examined under laboratory conditions. The supercooling points (SCPs) of pupae dropped significantly from a mean of -15.6°C in prediapause to -20.1°C in diapause, and then increased to -17.5°C during postdiapause development. Sex and voltinism influenced body mass but had no significant effect on the SCP. Our data demonstrated that cold tolerance of A. pernyi is tightly linked to life stage. Exposure of eggs to -5°C for up to 8 h had no effect on the hatching rate, whereas silkworm larvae failed to break through the chorion and hatch following a 4-8-h exposure to -10°C. Mean SCPs of intact eggs and naked larvae one day before hatching were similar, -23.3°C and -22.3°C, respectively, indicating that chorion does not significantly affect SCP. Comparison of lower lethal temperature (LLT50) and SCP means suggested that both pupae and eggs of A. pernyi are chill intolerant. These data will improve our understanding of low temperature tolerance in this commercially important species.