## 1 Introduction  
When importing and exporting grains, fruits, vegetables, and plants in the trade between countries, fumigation agents are normally used against quarantine pests [1, 2, 3]. Methyl bromide (MB) is the most versatile, and it has an excellent control efficacy against various insect pests, fungi, weeds, and nematodes [4, 5]. However, MB has been confirmed to contribute to ozone layer depletion, a major cause of global warming; therefore, its use has been banned and phased out from the market [6]. Although MB is still used in the quarantine and pre-shipment stages during trade, alternative fumigants are urgently needed as its use may be wholly prohibited soon [1; 3; 7].  
Many alternatives to MB have been tested, from physical control (e.g., heat, cold, and aridity), fumigant alternatives (e.g., phosphine, carbonyl sulfide, sulfuryl fluoride, carbon dioxide, ethyl or methyl formate, and so on), contact insecticides, and combination treatments (e.g., fumigation with low temperature, multiple fumigants) [8]. Based on previous studies of alternatives to MB, phosphine (PH3) is the most ideal fumigant in the world, but it has several drawbacks, including slow activity and increased insect resistance [8]. The emergence of PH3 resistance is a serious problem worldwide, especially in stored-grain insect pests [9; 10]. High resistance factors in the range of 60- to 600-fold based on median lethal concentration (LC50) values were reported in major stored-grain pests [11]. Therefore, studies on PH3-related resistance mechanisms, discovering the optimum concentration of PH3 for pest fumigation, and the development of combination fumigation methods with PH3 and EF have been widely conducted [11; 12; 13]. These various trials for solving the resistance problem are essential for prompt pest quarantine applications of PH3.  
Ethyl formate (EF) is a natural substance with relatively low toxicity to animals [14; 15] and has fast-acting properties similar to MB [16]. The resistance of insect pests to EF has not been reported, but EF has a relatively higher application concentration than PH3 [1]. There is currently no single alternative to MB's broad range of activities and benefits [8]. Therefore, the evaluation of various combination treatments and integrated pest management methods is required, along with toxicity mechanism studies to completely replace MB.  
The combination treatment of two fumigants has been reported from a variety of applications due to the complementary features; EF acts quickly at high doses and PH3 acts slowly at low doses [7; 17; 18]. To apply the combinational application in quarantine, pineapples and sweet pumpkins were used for controlling two-spotted spider mites and citrus mealbybugs in Korea [18; 19]. _Planococcus citri_ Risso (Hemiptera: Pseudococcidae) has been considered an economically important insect pest of various crops worldwide [20; 21]. In particular, it is registered in each country as a pest to be controlled in plant quarantine; thus, quarantine management should be systematically established for EF and PH3 fumigation. However, an understanding of the toxicity mechanism for EF and PH3 combination treatment against _P. citri_ has not been sufficiently studied.  
Thus, this study aimed to evaluate the efficacy of EF and PH3 treatments (alone and in combination) on all developmental stages of _P. citri_. The mode of action of combination treatment of EF and PH3 was verified using biochemical and molecular studies, including proteomics and lipidomics techniques.