Larval Dispersal of the Weed Biological Control AgentOxyops vitiosa(Coleoptera: Curculionidae)

Abstract

The Australian tree Melaleuca quinquenervia at both sites; no other larval stage were observed (Cav.) S. T. Blake (Myrtales: Myrtaceae) was inon the forest floor. troduced into Florida in the late 1800s, where While these data provide support for the hy it has naturalized and proven to be a superior pothesis that larvae freefall from canopies to competitor to most native vegetation. Florida forest floors, an alternative explanation for this M. quinquenervia wetland forests typically form phenomenon may be that 5th instar larvae are dense monocultures with continuous upper candisproportionately dislodged from the canopy and opies and low species diversity (Rayamajhi et al. subsequently recorded by observers. Therefore, a 2009). M. quinquenervia trees can exceed 30 m second method was employed in Feb 2003 that in height and tree architecture in dense stands is involved tracking larval dispersal in a more eas characterized by straight vertical trunks topped ily monitored environment. A 10 x 10 m area was by canopies restricted to the uppermost portions covered with a polyethelene plastic sheet that of the tree. Oxyops vitiosa Pascoe (Coleoptera: was staked to the ground. Five 2-m tall potted Curculionidae) is a biological control agent of M. quinquenervia trees were placed on top of the this exotic weed (Center et al. 2000). Weevil plastic sheeting and arranged to maximize spac larvae are specialized defoliators of M. quinqueing between individual trees in order to eliminate nervia foliage, consuming only expanding leaves any contact among their canopies. Ten late instar at branch apices (Pratt et al. 2004). Larvae exO. vitiosa larvae were placed in the canopies of ude a sticky orange secretion that covers the ineach tree. Immediately thereafter, the basal 10 tegument (Wheeler et al. 2002). This secretion cm of each tree trunk was coated with a layer of is often deposited on plant surfaces where imsticky material (Tanglefoot®) applied from an matures have crawled, leaving a trail. Larvae aerosol dispenser. Similarly, the sticky material complete 5 instars before seeking pupation sites was also applied to the surface of the plastic in a in the soil. When considering the heights of their 2 m radius around each tree. Searches for larvae host plants, we questioned how O. vitisoa larvae trapped on the sticky barrier at tree bases or in disperse from their feeding sites in the upper the sticky coating on the plastic commenced 24 h portions of the canopy to pupation substrates on later and were conducted twice daily (at 8 a.m. the forest floor. and 5 p.m.) for 2 wk. No larvae were found on the Two methods were used to investigate this basal barriers and no secretion trails led to these question. The first involved direct observation of areas. In contrast, 43 larvae were recovered on the forest floor and tree trunks for dispersing O. the plastic sheeting (86% recovery). Larvae were vitiosa larvae during the winter mo when larval found at both sampling times during the obser densities are highest. Observations were made at vation period, indicating that larval dispersal is 2 long-term M. quinquenervia research sites in not synchronized. Not surprisingly, larvae were Florida, one near Lake Okeechobee (N 26.784° W uniformly located within the drip line of study -80.950°) and the other along the Everglades buftrees. A small (~2cm) irregularly shaped yellow fer strip (N 26.050° W -80.433°). Both sites were secretion surrounded each larva, indicating the dominated by large M. quinquenervia trees growimpact point. The remaining 7 larvae were not ing in organic soils covered with thick layers of recovered and may have succumbed to predation leaf litter. A 2 h search was made during the last (Christensen et al. 2011). wk of Jan of both 2001 and 2002 by haphazardly Freefall of coleopteran larvae in search of suit reviewing stands for larvae moving down tree able pupation sites is a commonly assumed, but trunks or falling from the canopies. In addition, rarely observed, pathway for reaching the forest observers inspected trunks for dispersing larvae floor (Clark et al. 1998). Selection pressures that or trails of yellow secretions deposited by larvae may have influenced the evolution of this behavior while migrating downwards. These survey efforts include reducing the time required to reach pupa resulted in no evidence of larval dispersal via tion sites. Dropping limits energy expended in this this mode. Similarly, no direct observations were effort versus climbing down tree trunks. Expedit made of larvae dropping from M. quinquenervia ing larval dispersal to pupation sites also reduces canopies. By following the sound of objects strikexposure to natural enemies. Although O. vitiosa ing the ground, however, observers were occasionlarvae can survive the fall from M. quinquenervia ally able to locate fifth instars on the forest floor canopies, their mortality rates as a result of these