Phoresy by Hemisarcoptes (Acari: Hemisarcoptidae) on Chilocorus (Coleoptera: Coccinellidae): Influence of subelytral ultrastructure

Abstract

The non-phoretic stages of mites of the genus Hemisarcoptes are predators of the family Diaspididae. The heteromorphic deutonymph (hypopus) maintains a stenoxenic relationship with beetles of the genus Chilocorus. The mites attach to the subelytral surface of the beetle elytron during transport. There is variation in mite density among species of Chilocorus. Both Hemisarcoptes and Chilocorus are being applied to biological control programs around the world. The objective of this study was to determine whether subelytral ultrastructure (spine density) plays a role in the evolution of symbiosis between the mite and the beetle. The subelytral surfaces of 19 species of Chilocorus and 16 species of Exochomus were examined. Spine density was determined for five subelytral zones: (A) anterior pronotal margin, (B) medial central region, (C) caudoventral tip, (D) lateral distal margin, and (E) epipleural region. Spine density on the subelytral surface of Chilocorus and Exochomus was inversely correlated with the size of the elytron for all zones except Zone C. This suggests that an increase in body size resulted in a redistribution of spines and not an addition of spines. The pattern of spine density in Exochomus and Chilocorus follows a single size-density trajectory. The pattern of subelytral ultrastructure is not strictly consistent with either beetle phylogeny or beetle allometry. The absence of spines i s not correlated with either beetle genus or size, and species of either Chilocorus or Exochomus may be devoid of spines in any zone, irrespective of body size. A general difference between species of Chilocorus and Exochomus is the fact that while spine density in Chilocorus is clinal relative to the size gradient, Exochomus is dichotomous and likely to have either many spines or no spines in a particular zone. No species of Chilocorus was completely devoid of spines. Five species of Exochomus had n o spines at all, thus making it difficult to interpret the primary function of the subelytral spines in a general way. Within the genus Chilocorus,spine density may play a synergistic role in host association. Based on morphological evidence alone, these findings lead to the hypothesis that the species of Chilocorus that would be most conducive to biological control application in conjunctions with Hemisarcoptes would be: C. cacti, C. distigma, C. fraternus, C. orbus, C. tristis and to a lesser extent C. bipustulatus.