Lures significantly increase trap efficacy for targeted insect species. The commercial lures we utilized increase capture of Asian citrus psyllid by 3–4 times (Czokajlo et al. 2015), and brown marmorated stink bug by 120 + times (Weber et al., 2019). Lures can, however, have consequences on the capture rates of non-target taxa, and these impacts are not always predicted by taxonomic boundaries. For example, lures of the Noctuidae moth Helicoverpa armigera (Hübner) attract significantly more non-target moths, bees and ladybird beetles to traps (Spears et al., 2016). We found that tomato potato psyllid (TPP) was repelled by both ACP and BMSB lures, despite TPP and ACP both belonging to the superfamily Psylloidea. Psyllids representing the families Aphalaridae and Psyllidae were also repelled by ACP lures. Asian citrus psyllid belongs to the family Liviidae and is not represented by any known species in Western Australia, so the intrafamilial efficacy of the ACP lure remains unknown. Attraction or avoidance to lures corresponds to the chemicals used. The chemicals in ACP lures are a mixture of organic volatiles present in new flushes of citrus host plants, namely aphla-phellandrene, beta-phellandrene, beta-caryophyllene, gamma-terpinene, ocimene and terpineol (Setamou, 2018). As the chemicals originate from citrus, it follows that the lure should attract insects that feed on citrus, or other plants that produce similar organic volatiles, regardless of taxonomic identity. In our experiment, the only group attracted to the ACP lure were ‘other triozids’, which was driven predominantly by lilli pilly psyllid Trioza adventicia Tuthill, an introduced species from eastern Australia that feeds on Syzygium species (Francesco Martoni, Submitted 2021). The host plant families of T. adventicia and ACP are different (Myrtaceae and Rutaceae, respectively), but we surmise that the plant volatiles in the ACP lures must feature similar volatiles to those of Syzygium to attract the lilli pilly psyllid. Indeed, ACP lures include clove oil (Setamou, 2018) which originates from Syzygium (Cortés-Rojas et al., 2014), likely explaining the ACP lure attraction for T. adventicia. Furthermore, clove oil produces strong avoidance behaviour in TPP (Diaz-Montano & Trumble, 2013), illuminating why TPP may have been repelled by ACP lures.
Instead of plant volatiles, lures may be based on insect pheromones, which will have an even more unpredictable effect on bycatch. For example, the H. armigera lures mentioned above are based on pheromones, but can significantly increase ladybird beetle, bee and nontarget moth captures by 23%, 110% and > 2,000%, respectively (Spears et al. 2016). BMSB lures are pheromone lures and, while TPP was significantly repelled by the BMSB lure, psyllids other than TPP were attracted to BMSB lures, despite BMSB and psyllids belonging to different taxonomic suborders of true bugs (Heteroptera versus Sternorrhyncha). Delving into our psyllid data, high capture rates of a diverse composition of different species, mainly native Acizzia species, often dictated the pattern. In many cases, however, Ctenarytaina longicauda Taylor appeared to be driving the association on BMSB lured traps at tomato, and on sticky traps at citrus. Ctenarytaina longicauda is another recent arrival in Western Australia from eastern Australia (Francesco Martoni, Submitted 2021) and although Lophostemon confertus (R.Br.) is the known host plant, in Perth C. longicauda can be found in high abundances feeding on citrus (M.Moir pers. obs.). Why C. longicauda was not attracted to ACP lures, which are derived from the plant volatiles of fresh citrus growth (Setamou, 2018), remains unknown.
In some cases, species-specific lures will attract an unintended species within the same family. We found that BMSB lures attracted the pest stinkbug Plautia affinis (Dallas), the only known species of Plautia to occur in southwest Western Australia. Plautia affinis is a likely introduction from eastern Australia as records in Western Australia prior to (Cassis & Gross, 2002) are scarce, with (Froggatt, 1907) restricting species distribution to the state of New South Wales. The earliest specimens from Western Australia were collected by L.J. Newman in Geraldton (ICDB 2018), no date is listed, but would have been after his appointment as Government entomologist in 1918 and prior to his death in 1938. BMSB lures contain, in part as a pheromone synergist for BMSB, the pheromone of Plautia stali Scott (methyl (2E,4E,6Z)-2,4,6-decatrienoate), which is a sympatric species with BMSB in Asia (SUGIE et al., 1996). This component of the BMSB lures likely attracted P. affinis in our study, and other surveillance around Australia (Western Australia, M. Moir unpublished data; (Horwood et al., 2019). Moreover, despite the diversity of native pentatomids in Western Australia (approximately 137 described species; (Cassis & Gross, 2002) lack of captures suggest that other species weren’t attracted to the lures or sticky traps. Few stinkbugs are attracted to BMSB lures elsewhere, such as the pest Acrosternum hilare (Say) (Aldrich et al., 2009), and Euschistus servus (Say) (Cottrell et al., 2020) in the United States, but such examples are uncommon with most stinkbugs relatively lure specific (Tillman et al., 2010).
We found that combining lures will not counteract one another, nor will they have a cumulative impact on species examined here, supporting similar results for lures on BMSB and pest wood-boring beetles (Cerambycidae and Scolytinae: Curculionidae) (Chase et al., 2018), as well as Lepidoptera lures (Brockerhoff et al., 2013). For example, P. affinis was attracted to BMSB lures whether they were in combination with ACP lures or not, and although TPP was repelled by both lure types, combinations of these lures did not increase repellancy significantly. Unsurprisingly, we found that over time lure effectiveness declined as the chemical degrades, which has been an identified issue for lure longevity (Suckling, 2000); (Cottrell et al., 2020). The impact of the lures were most prominent for all taxa in the first two weeks, after which TPP was the only insect still detecting and avoiding BMSB lures.
While lures affected non-target insect capture in unpredictable ways, colour had a more consistent effect on Psylloidea. Yellow sticky traps are commonly used in surveillance as they have been found more effective at capturing Psylloidea than other colours tested such as white and black (Hodge et al., 2019). Specialist behavioural and anatomical studies have, however, indicated that Psylloidea have very good ‘colour vision’ (Farnier et al., 2015) and different species may prefer different colour spectrums (Farnier et al., 2014); Czokajlo et al. 2015). Despite this, we found that yellow-green ACP traps consistently attracted more Psylloidea, including TPP, other Triozidae, Psyllidae and Aphalaridae, than standard yellow traps. Conversely, stinkbugs were not attracted to either trap colour. Although our study had very low numbers of stinkbugs, this result is supported by (Cottrell et al., 2020) who found that BMSB and other stinkbugs are not significantly influenced by trap colour.
Our results provide evidence that olfactory cues are clearly more species-specific than colour for both Psylloidea and Pentatomoidea. This is unsurprising given that previous studies have shown the high sensitivity of olfactory cues, particularly for Psylloidea. For example, apples infected with apple proliferation phytoplasma (Phytoplasma mali) are more attractive to the vector summer apple psylla, thus facilitating the phytoplasma’s spread (Mayer et al., 2008). Likewise, citrus infected with Ca. Liberibacter asiaticus attracts the vector, Asian citrus psyllid (Mann et al., 2012), and tomatoes infected with Ca. Liberibacter solanacearum attract uninfected TPP, while infected TPP prefer uninfected plants (Mas et al., 2014). TPP may be particularly sensitive to aromas as it is repelled not only by ACP and BMSB lures, as examined here, but also by essential oils of cedar wood, lime, savory, thyme, tea tree ((Diaz-Montano & Trumble, 2013), garlic (Wright et al., 2013a), and different sulphur applications (Diaz-Montano & Trumble, 2013); (Wright et al., 2013b).