Delayed mortality, resistance and the sweet spot, as the good, the bad and the ugly in phosphine use

Evagelia Lampiri,Paraskevi Agrafioti,Christos G Athanassiou

doi:10.1038/s41598-021-83463-y

Abstract

Phosphine is the most commonly used gas for fumigation for durable commodities globally, but there is still inadequate information regarding its efficacy in conjunction with proper concentration monitoring. In a series of bioassays, insect mortality after specific exposure intervals to phosphine in selected species was examined, as well as the appearance of the so called "sweet spot". The species that were tested were: Oryzaephilus surinamensis (L.), Tribolium castaneum (Herbst), Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) with populations that had different levels of phosphine resistance. Evaluation was conducted by using the Phosphine Tolerance Test (PTT), with exposure of the adult stage for 15, 30, 60, 90, 150 and 300 min at 3000 ppm. At the end of these intervals (separate bioassays for each time interval), the insects were transferred to Petri dishes, in which recovery was recorded at different time intervals (2 h, 1, 2 and 7 days). The majority of susceptible populations of all species were instantly immobilized even in the shortest exposure period (15 min), in contrast with resistant populations that were active even after 300 min. After exposure to phosphine, populations and exposure time affected mortality of susceptible populations, whereas resistant populations recovered regardless of species and exposure time. Additional bioassays at the concentrations of 500, 1000, 2000 and 3000 ppm for 1, 3, 5, 20, 30 and 40 h showed the presence of the "sweet spot", i.e., decrease of mortality with the increase of concentration. In fact, for most of the tested species, the "sweet spot" appeared in 1000 and 2000 ppm at a 5-h exposure time, regardless of the level of resistance to phosphine. This observation is particularly important both in terms of the assessment of resistance and in the context of non-linear recovery at elevated concentrations, indicating the occurrence of strong hormetic reversals in phosphine efficacy.

Highlights

Phosphine is the most commonly used gas for fumigation for durable commodities globally, but there is still inadequate information regarding its efficacy in conjunction with proper concentration monitoring
For T. castaneum, there were some deviations in immobilization at longer exposures, due to a small number of adults that indicated some temporary movement (Fig. 1D)
Immobilization was low for the resistant populations; there was no immobilization of the resistant S. oryzae population, for the entire observation period (Fig. 1B)

Summary

Introduction

Phosphine is the most commonly used gas for fumigation for durable commodities globally, but there is still inadequate information regarding its efficacy in conjunction with proper concentration monitoring. Phosphine is currently the main gaseous insecticide that is applied for insect control in a wide range of durable commodities, such as cereals, legumes, oilseeds, tobacco, dried fruits and herbs, and used in food storage and processing facilities or transport vehicles[1,2,3,4,5] This gas is important for global food security, as it combines efficacy against a wide range of pest species that occur at the post-harvest stages of agricultural products and food, with low application c ost[6,7,8,9]. It is generally regarded that all these methods are mostly adapted for laboratory-based scientific research and have far less usefulness at the industrial scale, which requires a rapid (e.g. a same day) diagnostic tool[22]

Methods

Results

Conclusion