Estimating Population Change and Dispersal Activity of Spiders in an Agricultural Landscape

An unattended trap was designed to iample andretainspiders dispersing from agriculturalgrassland and crops. Traps comprised a removable bottle-trap fixed to the top of a vertical metal rod or"climbing-stick" that spiders climbed during normal pns-ballooning behavior. Bottle-traps caught overeight times more spiders than sticks treated with insect trapping adheslvie. Draping sticks with nets in­creased the effective area of the traps and increased the catch size threefold. On average, 9.1% of spiderswere lost from traps during the daytime sampling period. No difference in average rate of loss of spideisfrom the bottle-traps was observed between night and djorlight hours. The boltle-Unp design Is economicaland simple to construct, erect and operate. Continuous sampling also allows multiple traps to be usedsimultaneously in various locations.Kcj^rords: Aerial dispersal, sampling, botUe-trap, climblng-stick Aerial dispersal by ballooning is a key strat­egy in the life liistories of many spiders, es­pecially pioneers of disturbed, patchy liabitatsexemplified by linyphiids in agricultural land­scapes (Thomas et al. 2003a). Quantifying tliedispersal power of tliese species is a necessaryprerequisite for accurately modeling sparialpopulation dynamics and developing success­ful sustainable management strategies. Vari­ous techniques that acdvely or passively in­tercept airborne spiders have been used tomeasure aspects of aerial dispersal. For ex­ample: the use of nets and sticky traps to mea­sure aerial density at one or more altitudes(Greenstone et al. 1987; Greenstone 1991;Thomas et al. 2003b); manual collection fromfences, wire, or string to quantify numberspassing a point or line per unit time (Vugts &Van Wingerden 1976; Thomas et al. 2003b);or water traps to quantify depositionratesperunit area (Weyman et al. 1995: Tliomas &Jepson 1999). These mediods are either laborintensive, reqiure operator attendance, cannoteasily sample several locations at the sametime, or may be cumbersome or expensive.An altemative sampling method exploitsthe climbing behawor normally exhibited byspiders as a precursor to ballooning (Black-wall 1827): spiders climb to a high pointwhere a silk line can be produced above thesurrounding vegetation and where suitable at­mospheric conditions for successful balloon­ing are likely to occur (Suter 1999). Sticks,canes or similar objects inserted into theground, provide artificial platforms that standhigher than the surrounding vegetation. Spi­ders climbing and attempting to balloon fromthese can be observed, or caught and counted,to give a relative indication of ballooning ac­tivity over a given period. Thorbek et al.(2002), in a validation of diis technique, foundtiiat numbers of spiders observed climbing a30 cm stick correlated well with numbers ob­tained from an aerial suction trap. Using asimilar techm'que to sample several habitatsovertime,Duffey (1956) applied a tackj' ad­hesive to die tops of canes to trap climbingspiders. However, the adhesive was adverselyaffected by hot, cold or wet weather and be­came clogged vn\h winged insects duringsummer months.This paper describes and evaluates a novel