Adhesive Monitoring Traps as a Mortality Risk for the Common Pipistrelle in Mediterranean Olive Agroecosystems: First Evidence from Greece and Implications for Bat Conservation

Abstract

Yellow adhesive chromotropic traps are widely deployed across Mediterranean olive cultivation to monitor B. oleae population dynamics, yet their adhesive surfaces can also retain non-target organisms. Here we report the first documented entrapment of the common pipistrelle Pipistrellus pipistrellus (Schreber, 1774) on an adhesive monitoring trap in Greece, recorded on 15 May 2026 in a traditionally managed olive grove on Lesvos Island (North Aegean). The adult specimen (wingspan of 23.1 cm, body mass of 5.9 g) was independently identified by two specialist chiropterologists and confirmed dead by direct examination. The trap surface bore small-bodied Diptera belonging to groups known to occur in the diet of P. pipistrellus, raising the possibility that prey-associated cues may have contributed to the encounter. We argue that this record exposes an agroecological contradiction relevant to conservation-oriented olive management: the monitoring device deployed to track B. oleae may incidentally eliminate an insectivorous bat belonging to the broader natural enemy community associated with olive agroecosystems. Given the extent of adhesive trap deployment across Mediterranean olive cultivation and the strict legal protection afforded to all European bats under the Habitats Directive (92/43/EEC), systematic bycatch recording, mechanistic investigation and evaluation of simple mitigation measures are warranted.

Traditional Mediterranean olive groves provide structurally heterogeneous habitat for diverse vertebrate fauna [1], including insectivorous bats that exploit their open-to-cluttered canopy airspace, linear foraging elements, and diverse nocturnal prey assemblages [2]. Among them, the common pipistrelle Pipistrellus pipistrellus (Schreber, 1774), broadly distributed across mainland Greece and most island groups [3], is consistently among the most active bat species recorded in Mediterranean olive farms [4]. As aerial insectivores, bats can exert measurable predation pressure on olive-associated insects, including the olive fruit fly, Bactrocera oleae (Rossi), the primary biotic constraint on Mediterranean olive production [5].

Yet the same olive systems used by foraging bats are routinely monitored with yellow adhesive chromotropic traps deployed to track B. oleae population dynamics [6], creating potential for unintended vertebrate bycatch. Ancillotto et al. [7] compiled 222 bat entrapments on adhesive traps across Europe encompassing 17 species between 1998 and 2023; bat bycatch in other passive entomological devices further attests to a broader vulnerability of insectivorous bats to monitoring infrastructure [8]. Viewed within the broader conservation context outlined by Frick et al. [9], such incidents may represent an overlooked form of anthropogenic mortality acting alongside the multiple pressures already affecting bat populations worldwide. Critically, records from the eastern Mediterranean remain scarce leaving their frequency and conservation significance poorly resolved.

Here, we document the first incident, to our knowledge, of P. pipistrellus entrapment on an adhesive monitoring trap in Greece, from a traditionally managed olive grove on Lesvos Island, North Aegean. By situating this observation within its documented trap-deployment context, we consider the mechanisms that may have led to entrapment, revealing a conservation paradox: a device deployed to monitor a major agricultural pest may itself create mortality risk for a protected insectivorous bat.

On 15 May 2026, during collection of adhesive panels deployed for flying-insect monitoring, a member of our team (K.K.) documented a dead adult bat attached to a yellow chromotropic panel in a traditionally managed olive grove in the central part of Lesvos Island, North Aegean, Greece. Direct examination confirmed that the individual was dead before the panel was repositioned for photographic documentation (Figure 1).

The bat was captured as part of a flying-insect monitoring scheme initiated in April 2026, in a grove which was managed under low-intensity conditions, with limited agrochemical inputs and periodic mechanical cutting of the understorey (Figure 2). Yellow chromotropic adhesive panels measuring 10 × 23 cm were deployed without chemical attractants at a density of three panels per hectare and suspended from olive branches at approximately 1.9–2.0 m above ground. In each monthly sampling round, the panels were exposed for seven consecutive days and subsequently removed and examined; no panels remained in the field between rounds. The bat was recovered during the second monthly round in May from the second panel examined during collection. Accordingly, four panels had been examined before the observation, and the bat was encountered on the fifth panel examined since monitoring began, following two seven-day deployment periods, equivalent to 14 calendar days of field exposure.

At the time of inspection, the panel had accumulated small-bodied Diptera (Ceratopogonidae, Sciaridae, Phoridae, Hybotidae, Psychodidae, Muscidae, Anthomyiidae), including several groups reported in the diet of P. pipistrellus [10]. Following removal from the panel, the specimen was independently examined for taxonomic identification by two specialist chiropterologists, while standard external measurements and post-mortem body mass were recorded from the specimen.

The specimen was identified as an adult P. pipistrellus (Schreber, 1774; wingspan of 23.1 cm, post-mortem body mass of 5.9 g). The individual was confirmed dead by direct examination before photographic documentation, as it was extensively adhered to the panel surface, with its wings spread and the patagium in direct contact with the glue layer. To our knowledge, this constitutes the first documented record of bat bycatch on an adhesive monitoring panel in Greece and one of the few reported from the eastern Mediterranean [7].

A deployment height of 1.9–2.0 m was used when placing the trap within the foraging and commuting flight stratum of P. pipistrellus in cluttered edge environments [11,12], and the mid-May timing coincides with both peak bat foraging effort following spring emergence [13] and the onset of the B. oleae monitoring season [14]; this phenological alignment may increase encounter probability, although the magnitude of this risk cannot be inferred from a single observation.

Within this context, two non-exclusive capture pathways merit consideration. Passive echolocation failure—a bat unable to resolve the flat adhesive panel in time to execute avoidance—is plausible, on two grounds. First, smooth flat surfaces behave as acoustic mirrors: when insonified at an oblique angle, they reflect most of the incident energy away from the emitting bat returning stronger echoes mainly at near-perpendicular incidence and potentially leaving insufficient time for avoidance [15]. Second, acoustic clutter from the surrounding olive canopy could further reduce the probability of timely detection and avoidance [16].

Another plausible pathway is active prey-associated attraction: the trap surface bore a visible accumulation of Ceratopogonidae, Sciaridae, Phoridae and Muscidae—families within the core dietary spectrum of this species [10] and dominant in the arthropod assemblages of low-intensity Lesvos olive groves [17]—constituting a concentrated prey patch potentially detectable through insect volatiles and acoustic emissions of struggling prey. The yellow colour of the panel functions as a visual attractant for insects; any attraction of the bat would therefore have been indirect and mediated by prey-derived cues. A bat investigating such a patch and contacting the adhesive surface may have approached the panel in response to these cues, although this mechanism remains hypothetical because the bat’s approach was not observed.

That pathway, if confirmed, would expose a feedback not previously explicitly incorporated into the bat bycatch literature. The individual documented here belonged to an insectivorous species potentially contributing to arthropod suppression in olive agroecosystems, yet was eliminated by a device deployed to monitor a major insect pest. Pipistrelle bats consume small-bodied Diptera [10], and bat insectivory is recognized more generally as an ecosystem service in farmland [18]. However, direct dietary evidence for Tephritidae, and specifically for B. oleae, in P. pipistrellus is currently lacking, so consumption of the olive fly itself is therefore inferred from prey-size compatibility. Whether adhesive traps remove foraging bats at rates sufficient to impair this potential pest-suppression function remains unknown because European records derive predominantly from opportunistic reports [7] and routine B. oleae surveillance does not systematically document vertebrate bycatch. Resolving this uncertainty requires standardized bycatch records linked to trap density, deployment duration, and inspection effort. Until such evidence becomes available, the conservation significance of the present record rests on the documented mortality of a strictly protected species caused by a monitoring device used in Mediterranean olive cultivation, irrespective of the strength of the inferred dietary link. This paradox extends beyond adhesive traps, as other routine agricultural practices and equipment may also generate unintended vertebrate mortality. [19] provides a comparable example of unintended vertebrate mortality associated with routine olive-management equipment.

This shift from habitat-level value to object-level risk is especially important because P. pipistrellus is protected under Annex IV of the Habitats Directive (92/43/EEC) and Appendix II of the Bern Convention, while its low reproductive output increases its vulnerability to recurrent additional mortality [20]. Although a single record cannot establish population-level risk, the cumulative effects on bat populations are entirely unquantified. Systematic incorporation of bat bycatch recording into routine trap inspections, a low-cost addition to existing protocols, would generate the prevalence data needed to move from isolated observations to quantitative risk assessment; if recurrent bycatch is demonstrated, modified trap placement, physical barriers, and temporal adjustments could then be evaluated as potential mitigation measures without compromising B. oleae monitoring.