Nectary use for gaining access to an ant host by the parasitoid Orasema simulatrix (Hymenoptera, Eucharitidae)

Abstract

Eucharitidae is the only family of insects known to specialize as parasitoids of ant brood. Eggs are laid away from the host onto or in plant tissue, and the minute first-instars (planidia) are responsible for gaining access to the host through some form of phoretic attachment to the host ant or possibly through an intermediate host such as thrips. Orasema simulatrix (Eucharitidae: Oraseminae) are shown to deposit their eggs into incisions made on leaves of Chilopsis linearis (Bignoniaceae) in association with extrafloral nectaries (EFN). Nectary condition varies from fluid-filled on the newest leaves, to wet or dry nectaries on older leaves. Filled nectaries were about one third as common as dry nectaries, but were three times as likely to have recent oviposition. Larger numbers of undeveloped eggs, or eggs with mature planidia inside, were associated with filled and wet EFN. For emerged planidia, the distribution was shifted from a concentration at filled nectaries to an even greater concentration at wet nectaries. More planidia were found in EFN (9.50 ±2.85) than outside EFN (1.00 ± 0.60). Planidia were tested for their attachment to adult and larval ants and to adult and immature thrips (potential intermediate host), but the results do not support simple attachment as a viable means for transfer and successful parasitism. Pheidole desertorum was identified as the host ant, and at night is the dominant ant in the tree canopy of C. linearis. Feeding at the EFN by the host ant, and the direct association with planidia near to or in the EFN, is interpreted as a novel means of accessing the host brood.