The attachment and stylostome of Trombidium newelli (Acari: Trombidiidae), an ectoparasitic mite on adults of alfalfa weevil, Hypera postica (Coleoptera: Curculionidae)

Abstract

Larvae of the mite Trombidium newelli Welbourn and Flessel are ectoparasitic on adult alfalfa weevil, Hypera postica (Gyllenhall), an insect pest of alfalfa. The mite larvae are found under the elytra, attached to the dorsal surface of weevils’ abdomen. T. newelli larvae use their chelicerae to penetrate the pliable and weakly sclerotized areas of the host’s integument, and to hold on firmly to the host. The attachment sites associated with larval mites of different degrees of engorgement were examined using both light and scanning electron microscopy. The ventral inside surface of parasitized host tergites revealed a characteristic injury as spots varying in color from white-yellow to golden-brown depending on the engorged size of the mite. In addition, spots associated with fully engorged mite larvae showed an aggressive dendritically radiating mass expanding outward from the site of cheliceral penetration. This mass, known as feeding tube or stylostome was branched, with each branch ending in a cluster of closed bulbs. Stylostomes appear to exist independently in the host’s tissue. Stylostome mass corresponded with the engorgement state of the mite, increasing as the mite larva increased in size. The possible nature and mechanism of stylostome formation is discussed.