Pre-exposure of developmental stages of Mediterranean flour moth to ultrasonic waves and its effect on lethal and sublethal effects of low temperature

Ultrasonic waves are mechanical waves that can adversely affect insects' biological and reproductive characteristics. On the other hand, low temperatures are regarded as a non-chemical and residue-free method for controlling stored-product insects because they affect the growth, development, population parameters, survival, reproduction, and various physiologic aspects of insects. In this research, due to the delayed effects of low temperatures, we examined the influence of pre-exposure of different developmental stages of the Mediterranean flour moth, Ephestia kuehniella Zeller, to ultrasonic waves on the lethal and sub-lethal effects of 4 °C temperature against emerged adults. The study's main goal was to mitigate the delayed effects of low temperatures.
 

All experiments were conducted at the Entomological Research Laboratory within the Department of Plant Protection at the Gorgan University of Agricultural Sciences and Natural Resources. The insect utilized for the research was reared on wheat flour at room conditions. Ultrasonic waves (20-60 kHz) were applied to eggs, first- and fifth instar larvae, and pupae. In addition, the following developmental stages were continuously exposed to ultrasonic waves: 4 days: egg pre-exposure; 9 days: eggs and first-instar larvae; 14 days: eggs to second-instar larvae; 19 days: eggs to third-instar larvae; 24 days: eggs to fourth-instar larvae; 33 days: eggs to fifth-instar larvae; and 40 days: eggs, five larval instars, and pupae. After pre-exposure treatments, emerged adults were exposed to 4 °C. Furthermore, LT50 and LT95 values were estimated for adults. Moreover, the sublethal effects of low temperature on male and female longevity, the fecundity of newly-emerged adults, and the percentage of egg hatches were investigated.

According to the results, pre-treatment of Mediterranean flour moth eggs with ultrasonic waves for 3 and 4 days significantly reduced the LT50 and LT95 of the exposed eggs when exposed to low temperature. In addition, pre-treatment of first and fifth instar larvae to ultrasonic waves did not reduce LTs of low temperature on emerged adults, whereas pre-treatment of pupae for eight days and consecutive pre-treatment of different developmental stages for 24, 33, and 40 days reduced LTs of low temperature significantly against emerged adults. In this study, although low temperature ultimately caused the death of adult insects, it increased the adults' longevity from 8-10 days up to 28-46 days. Furthermore, no significant effect of pre-treatment of eggs and first instar larvae with ultrasonic waves was observed on the longevity, fecundity and percentage of egg hatch of emerged adults. Moreover, pre-treatment of fifth instar larvae and pupae had a noticeable effect on adult longevity and fecundity, but did not affect egg hatchability. In all cases, pre-treatment of newly emerged adults with low temperature had a major effect on these traits. According to the findings, the longevity of male and female adults exposed to low temperature was significantly shorter than that of the control group following their release from these conditions. The results demonstrated that low temperature reduced pre-treated adult insects' fecundity and egg- hatching rate.

The current study's findings amply demonstrated that pre-exposure of some developmental stages (for example, larvae and pupae) of the Mediterranean flour moth could reduce LTs of low temperatures against emerged adults while also increasing the negative effects of these temperatures on their longevity, fecundity, and percentage egg hatch. This observation can be used to control stored-product insect pests.