Parasitism of Spissistilus festinus (Hemiptera: Membracidae) by Halictophagus jordani (Strepsiptera: Halictophagidae) in Arizona alfalfa fields

Sweep net samples taken from alfalfa fields in Arizona contained 3-cornered alfalfa hopper, Spissistilus festinus (Say), abundantly stylopized by an unknown strepsipteran parasitoid. Targeted sampling of S. festinus was conducted over a 2-wk period from October to November to determine the rate of parasitism and identify the parasitoid. Adult male parasitoids were reared for morphological identification. Additionally, representative male, female, and larval parasitoids were identified through COI barcoding gene sequencing. Strepsiptera were determined to be Halictophagus jordani (Pierce), previously reported only from eastern Texas and Louisiana, United States. All adult S. festinus and mature H. jordani were sexed and counted, and all apparently unstylopized hoppers were dissected to look for internal parasitoid larvae. Parasitism rates, parasitoid loads, and parasitoid sex ratios of both host sexes were compared. Overall, of 3,791 S. festinus examined, 1,092 (28.8%) bore mature H. jordani protruding between abdominal segments. A further 613 (16.2%) were found to contain internal larvae upon dissection, yielding a total stylopization rate of 45.0%. Male and female hoppers did not differ significantly in mature male, female, or total parasitoid load, though female hoppers were significantly more likely to be parasitized than male hoppers. Parasitoid infrapopulations showed evidence of segregation by parasitoid sex, but no relationship was detected between infrapopulation sex ratio and host sex. The size of an infrapopulation strongly predicted its sex ratio, which became more male-skewed with increasing size. These findings indicate that H. jordani is more widely distributed than previously believed and suggest its potential role in the biological control of S. festinus.