Characterization of Chemical Defensive Behavior and Associated Glands in the Destructive Invasive Longhorn Beetle Aromia bungii

TL;DR

This study reveals how the invasive red-necked longhorn beetle defends itself using specialized glands and a spray mechanism, offering insights into its survival and reproduction strategies.

Contribution

The first detailed characterization of a chemical defense system in the longhorn beetle Aromia bungii, including its morphology and function.

Findings

Aromia bungii has triangular sac-like glands in the metathorax that eject defensive substances through small openings.

The beetle's defensive spray contains significant amounts of substances, but these are not quickly replenished after use.

The chemical defense system reflects an energy trade-off between reproduction and defense in this short-lived species.

Abstract

The red-necked longhorn beetle Aromia bungii is a destructive invasive pest that causes substantial damage to economically important stone fruit trees such as cherries and plums. While chemical defenses are common in insects, they are rarely reported and remain poorly understood in longhorn beetles (Cerambycidae). This study aimed to characterize the chemical defense system of this species for the first time. Using advanced imaging techniques, we identified a pair of unique, triangular-shaped, sac-like glands storing liquid defensive substances, located in the beetle’s metathorax. When threatened, the beetle rapidly ejects this liquid over a long distance through a pair of tiny openings on its body. A single spray contains a substantial amount of defensive substances. However, the beetle probably cannot quickly replenish these substances, because in a second spray performed after 10 days only a small amount was ejected. This indicates that the beetle may prioritize energy for reproduction over repeated syntheses of defensive substances. Understanding this highly specialized defense mechanism provides insights into this beetle’s survival and reproduction strategies which are valuable for developing novel strategies to manage this pest.

This study characterizes the chemical defense system of the invasive longhorn beetle Aromia bungii, a destructive pest of Prunus trees, addressing the limited understanding of chemical defensive mechanisms in Cerambycidae. High-speed cameras, environmental scanning electron microscopy (ESEM), dissection, and micro-CT imaging were used to investigate defensive behavior, and the structure of the defense system, in this beetle. Both sexes of A. bungii possess a pair of triangular, sac-like defensive glands symmetrically located in the metathorax, attached to the metasternum. Upon mechanical stimulation, white liquid defensive substances are rapidly ejected through a pair of slit-shaped openings (~200 µm) at the metasternum corners, without gland eversion, reaching over 50 cm. The average weight of substances ejected in first sprays was 7.95 ± 0.79 mg for females and 8.62 ± 2.13 mg for males (mean ± se), with no significant difference between sexes. However, the weight in second sprays after 10 days was significantly lower, at 2.93 ± 0.54 mg for females and 2.22 ± 0.40 mg for males (mean ± se), suggesting that the beetles cannot re-synthesize the substances soon after spray. The weight of ejected substances had no correlation with beetle body weight. Our findings represent the first detailed morphological and functional description of a chemical defense system in Cerambycidae, revealing a specialized metasternal gland and spray mechanism. The substantial but likely non-renewable defensive substances reflect an adaptive trade-off in energy allocation between reproduction and defense in this species that exhibits high fecundity but a short lifespan at the adult stage.

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