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, o r 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. Mo st 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 approache d, contributed significantly to the mortality if the rate of heating was re latively slow. Calculations of egg survival in a mathematical model of the conditioning and lethal thermal responses, correlated well with experimenta l values in terms of both trends and magnitudes of LT99 values. The thermal conditions, prior to disinfestation treatment, influence the response to s ubsequent heat treatment and thus have implications for the specification o f postharvest quarantine treatments which are often expressed in terms of a fruit centre target temperature. This does not take into account of the in fluence of temperatures and exposure times in the range 32-42 degreesC whic h can have a very significant effect on the time required to reach high lev els of mortality. An efficacious treatment may be the combination of a leth al 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 a ssist in the identification of promising treatments while avoiding extensiv e in-fruit testing. (C) 2000 Elsevier Science B.V. All rights reserved.