What is the Maximum Foraging Depth of Cricotopus lebetis to Mine the Apical Stems of the Noxious Aquatic Weed Hydrilla?

Cricotopus lebetis Sublette (Diptera:  Chironomidae) attacks the apical meristems of Hydrilla verticillata (L.f.) Royle   commonly called hydrilla.  Hydrilla is listed as a noxious aquatic weed in Florida and the U.S.A.  Hyrilla damaged by C. lebetis is sometimes unable to grow to the surface.  As a result this Florida endemic insect shows promise as a biological control agent.  One larval C. lebetis per shoot tip mine and feed on the vascular tissues of the apical meristems of hydrilla damaging it and preventing it from reaching the surface thereby changing the plant architecture.  The neonate C. lebetis free-swimming and vulnerable to predation, but their translucent color and small size may afford them some protection until they can enter a shoot tip.  This study was conducted in part to determine the maximum foraging depths by larval C. lebetis.  Plants were placed in 10 cm diameter tubes at depths of 3 m, 2.3 m, 1.5 m and 0.8 m (4 replicates at each depth).  Neonates were placed in each tube and evaluated for the ability to reach and attack the hydrilla plants at that depth.  The depths chosen were based on depths at which hydrilla occur in certain spring fed rivers in north Florida.