## 1 Introduction  
Asian longhorned beetle _Anpolophora glabripennis_ (ALB) adults emerge from June to August to supplement nutrition, mating, and oviposition in northern China. Adults gnaw notch grooves on host tree stems and ovip posit one egg within each groove, and the hatched larvae eating the phloem and xylem severely destroy the physical structure and physiological function of the tree, which can cause the whole tree to die. Its host species are abundant and mainly include _Acer_, _Populus_, _Salix_, _Ulmus_, _Betula_, etc., [1, 2, 3]. The northwestern area in China is in arid and semi-arid region, where only a few tree species are able to be planted as a windbreak shelterbelt forest. _Populus alba_ var. _pyramidalis_ (PaP) is the main tree species found there because of its high height, narrow crown, salt and alkali resistance, and resistance to ALB to some extent. However, PaP can also be infested by ALB when there is no sensitive tree, so it is urgent to find effective methods to protect PaP [4, 5]. Plants have evolved a complex defense system during the process of long-term plant and insect coevolution, including both constitutive and inducible defenses [6]. Upon insect infestation, the expression of a series of resistance genes is initiated through the plant's hormone system, including the production of secondary defense substances and insect-resistant defense enzymes [7, 8, 9, 10, 11, 12]. Jasmonic acid (JA) is a plant hormone that can be catalyzed by a series of enzymes to generate methyl jasmonate (MeJA). Mutual transformation of JA and MeJA results in signal transmission, transcription regulation, expression of related resistance genes, and synthesis of biochemical substances, thereby improving plant defense systems and resistance to infestation [13, 14, 15, 16]. Salicylic acid (SA) is another important endogenous signal molecule responsible for a series of defense responses to diseases and pests. SA and SA-binding proteins can induce plant systems to exhibit system acquired resistance (SAR) that enhances innate plant resistance [17, 18]. In recent years, studies have found that plants can form an immune memory called defense priming, and thus, exposure to trigger stimuli can respond faster or stronger to subsequent attacks of pests and pathogens. The initiation of this inducible defense provides increased protection and reduces the allocation cost of defense [19]. Application of plant hormones or other related substances like MeJA can induce or activate related signal transduction in plants, thereby improving the ability of plants to resist phytophagous insect damage [20, 21, 22]. We hypothesized that exogenous plant hormones could also trigger the defense system of PaP against ALB. Volatiles emitted by host plants can mediate interspecific interactions between plants and insects [23]. Our previous results showed that (Z)-3-hexenol (Z3H) and (Z)-3-hexen-1-yl acetate (Z3 HA) released from PaP were attractive to ALB [3, 5]. We hypothesized that both volatiles might interact with the defense system of PaP, so in this study, we first analyzed the quantities of Z3H and Z3 HA in the branches and leaves of healthy, mechanically damaged, and ALB-infested PaP. It is surprising that the quantities of the two volatiles from the latter two treatments were significantly lower than the healthy one, especially in the ALB-infested treatment. We investigated the synthesis pathways of Z3H and Z3 HA through the Kyoto Encyclopedia of Genes and Genomes (KEGG) and found that both volatiles showed an antagonistic relationship with the synthesis of JA and SA. Then, we determined the contents of SA, JA, methyl salicate (MeSA), and MeJA in the phloem of PaP either on healthy or ALB-infested conditions and found that both JA and MeJA were actually involved in the ALB-resistant response of PaP as a positive regulatory factor, especially MeJA.