Thermal conditioning in Bactrocera tryoni eggs (Diptera: Tephritidae) following hot-water immersion

Bactrocera tyroni Froggatt eggs were immersed in hot water to determine egg mortality. Eggs were either immersed in water at a constant temperature, or experienced changing temperature at a specified rate of increase and from a specified start temperature. Comparison of the estimated lethal time for 99% kill (LT99) of eggs experiencing different treatments allowed thermal conditioning to be identified and quantified. Conditioning depended on the temperature and duration of treatment, being a maximum near 38 degreesC. Most conditioning appeared to occur during the early part of the exposure to a given temperature. Exposure to lethal temperatures (greater than or equal to 42 degreesC) as a target temperature of 46 or 48 degreesC was approached, contributed significantly to the mortality if the rate of heating was relatively slow. Calculations of egg survival in a mathematical model of the conditioning and lethal thermal responses, correlated well with experimental values in terms of both trends and magnitudes of LT99 values. The thermal conditions, prior to disinfestation treatment, influence the response to subsequent heat treatment and thus have implications for the specification of postharvest quarantine treatments which are often expressed in terms of a fruit centre target temperature. This does not take into account of the influence of temperatures and exposure times in the range 32-42 degreesC which can have a very significant effect on the time required to reach high levels of mortality. An efficacious treatment may be the combination of a lethal stress and a particular heating rate which falls within a band, bounded by rates of heating that are too slow or too fast. The use of models will assist in the identification of promising treatments while avoiding extensive in-fruit testing. (C) 2000 Elsevier Science B.V. All rights reserved.