Quantity Does Matter: How Feces Are Used for Host Stage Selection by Granary Weevil Parasitoid Lariophagus distinguendus

Abstract

The present paper examines the mechanisms of host stage selection in Lariophagus distinguendus, a larval parasitoid of the granary weevil Sitophilus granarius endophytic in wheat grain. The amount of host feces accumulating in infested grains was on average, 0.2, 0.3, 1.5, and 4 mg for first, second, third, and fourth instars, respectively. A maximum of 9 mg feces was found in grains with fourth instars. In bioassays with female parasitoids, only a little drumming and drilling behavior was observed on grain models treated with feces extract equivalent to 0.2 mg and 0.4 mg feces. Most drumming and drilling occurred on models with extracts equivalent to 4 and 8 mg feces. These results correspond to literature data demonstrating that older host larvae (fourth instars) are preferred for oviposition. Thus, host stage selection in L. distinguendus is apparently achieved by assessing the quantity of feces present in infested grains.