Vibrational signals differ between contests and copulatory courtship in the red milkweed beetle Tetraopes tetrophthalmus (Coleoptera: Cerambycidae)

Abstract Substrate-borne vibrational communication is commonly used by herbivorous insects to communicate intraspecific information. This form of communication is underexplored and many discoveries of the form and functional significance of these vibrations have yet to be made. Here, we add red milkweed beetles, Tetraopes tetrophthalmus (Coleoptera: Cerambycidae) to the record of arthropods that communicate with these plant-borne vibrations. Red milkweed beetles, like other cerambycids, commonly use stridulation to produce audible squeaks when in stressful situations and can also produce plant-borne vibrations when in contact with conspecifics. These beetles engage in fights for territory which ultimately results in mating opportunities (i.e., contests). We placed beetles into three sex-ratio treatments: male–male (contests), male–female (copulatory courtship), or male–male–female (contests and copulatory courtship). We first describe contests and copulatory courtship behaviours. We then used laser vibrometry and found that red milkweed beetles communicate by producing plant-borne vibrational signals composed of low-frequency rumbles and high-frequency clacks during male–male contests and copulatory courtship. We compared the spectral and temporal features of the vibrations between the two pre-copulatory contexts. We found that vibrational signals were shorter and had a higher fundamental and dominant frequencies when beetles engaged in contests compared to copulatory courtship behaviours. Our results show that these beetles use plant-borne vibrations as context-dependent signals that communicate different information under these two pre-copulatory scenarios. Our data suggests that these context-dependent signals may help beetles gain access to mates and prolong copulation.