5.6. Solve Issues of Factors Affecting Efficacy
Of the various factors that have been hypothesized as affecting the efficacy of PI [32], oxygen content is the one that has already impacted regulation of the treatment. Irradiation under hypoxic conditions may reduce efficacy. Therefore, plant protection organizations have prohibited the use of PI on commodities stored in hypoxic atmospheres if the research supporting its use under hypoxic conditions has not been done [27,33]. Although some research has shown that this is not a problem for tephritid fruit flies [34,35] definitive research is needed to put to rest this issue for this important group of quarantine pests. Additionally, some research was done using larvae reared on diet and inserted into holes bored to the centre of fruits [35], which presents a series of untested assumptions concerning results of these studies. Regardless of the final conclusion for fruit flies, irradiation in hypoxic atmospheres seems more of a concern for most other quarantine pests, which seem to respond to hypoxia to a greater extent than fruit flies [36]. Another issue that is of concern is whether temperature during irradiation affects efficacy. It does not seem to be the case [32], but further research may be warranted before putting the issue to rest. The two main physical characteristics that differentiate gamma from electron beam irradiation are depth of penetration and dose rate; electron beams have low penetration but deliver very high dose rates whereas gamma rays are penetrating and have relatively low dose rates. Dose rate is hypothesized to directly affect PI efficacy; i.e. ,a faster dose rate may lead to increased efficacy because it over whelms radiation damage repair mechanisms. High dose rates also result in oxygen depletion and the presence of oxygen significantly increases the efficiency and nature of radiation damage. Therefore, dose rate effects are especially of interest because electron beams are increasingly being used in PI and most research was previously done with slower gamma sources. Fresh commodities and insect pests could conceivably have different tolerances to fast dose-rate sources but there are few studies in this area and there is no evidence of the effect being significant at the dose rates used commercially.