Differentiating between male and female stink bugs, such as the common Brown Marmorated Stink Bug (Halyomorpha halys), requires close examination. Unlike many insects with obvious sexual dimorphism, the differences are anatomical and minute, concentrated in the structures of the abdomen. These subtle distinctions necessitate a focused examination to determine the sex accurately. The primary method for reliable sexing involves observing the shape and structure of the final abdominal plates on the insect’s underside.
Preparing for Close Examination
Since the defining characteristics are located on the ventral, or underside, of the insect, the specimen must be oriented correctly for observation. Stink bugs must be carefully immobilized or placed on their backs to expose the abdomen. This process requires gentle handling to avoid causing the insect to release its defensive odor.
The minute scale of the differences means that the unaided eye is insufficient for a definitive determination. High magnification is necessary, often requiring a strong hand lens or a dissecting microscope. Proper, bright lighting is crucial to clearly illuminate the fine lines and contours of the last abdominal segments. The successful identification depends entirely on clearly viewing the terminal plates, which are small even on a full-grown adult.
Identifying Sex by Terminal Abdominal Plates
The most reliable indicator of a stink bug’s sex lies in the morphology of the terminal abdominal sternites, which are the rigid, plate-like segments on the ventral side. These plates form the external genitalia, which are distinctly different between the sexes. The final segment of the abdomen in the male Halyomorpha halys typically appears flatter and more uniform.
In males, the last visible abdominal segment is generally narrower and appears simple or somewhat rounded. This terminal plate may feature a slight “rear ventral scoop” or contain the male claspers, which are external structures used during mating. The overall look is that of a single, less-divided final structure.
The female terminal segment, in contrast, is noticeably modified to accommodate the ovipositor. The last abdominal segment of the female is usually broader and larger than the male’s, providing space for this reproductive equipment. This terminal plate often appears divided or possesses a distinctive notch or a U-shaped plate at its posterior edge. This division gives the female’s terminal abdomen a more complex shape compared to the male’s rounded final plate.
Observable Differences in Size and Shape
While the terminal abdominal plates offer the definitive method for sexing, female stink bugs often tend to be slightly larger than males. This difference is likely related to the energy demands of egg production.
This small size discrepancy, however, is not a dependable tool for individual identification because the size of any stink bug can vary significantly based on its diet, environment, and overall health. Subtle differences in the width or shape of the thorax or head region are also highly variable and poor indicators of sex. Therefore, these size and shape differences should only be considered suggestive and secondary to the examination of the abdominal sternites.