Clover in vineyards, a potential trap plant for the mealybug <italic>Pseudococcus calceolariae</italic>—a vector of GLRaV-3 to grapevines but not clover species.

Grapevine leafroll disease (GLD) affects grapevines worldwide. The primary causal agent of GLD is grapevine leafroll-associated virus 3 (GLRaV-3), which spreads to uninfected grapevines via mealybugs and soft-scale insects. <italic>Pseudococcus calceolariae</italic> (Hemiptera: Pseudococcidae) is a mealybug vector of GLRaV-3 in New Zealand. <italic>P. calceolariae</italic> also colonizes clovers (<italic>Trifolium</italic> spp.) growing naturally as vineyard ground cover. Separating mealybug from GLRaV-3 grapevine host could be enhanced by a trap plant: an alternative host attractive to and retentive of the target pest. We evaluated the association between <italic>P. calceolariae</italic> and ‘Grasslands Huia’ white clover (GHWC). GHWC seed was sown under grapevines in a commercial vineyard (14 × 0.4 ha plots); the control was under-vine herbicide use (7 × 0.4 ha plots, where only few <italic>Trifolium</italic> spp. plants grew). After 2 years, GHWC cover peaked at 40% mealybug infestation in 2019. From 2018 to 2021, <italic>P. calceolariae</italic> detection and abundance on GHWC was significantly higher than plants from the control plots. There was no treatment effect for mealybug infestation of grapevine leaves nor of GLRaV-3 incidence, independent of vintage. A glasshouse trial found no transmission of GLRaV-3 by <italic>P. calceolariae</italic> to any of 256 plants among five clover cultivars tested (<italic>Trifolium</italic> spp.), including GHWC; mealybug transmitted GLRaV-3 to 35 of 107 <italic>Nicotiana benthamiana</italic> plants. The results showed that in the 5-year period, added GHWC did not decouple <italic>P. calceolariae</italic> from the grapevine to reduce GLRaV-3 incidence, but rapid colonization of GHWC by mealybug and the lack of GLRaV-3 transmission to GHWC are encouraging. Further evaluation is needed to assess whether plant biodiversity can benefit a GLRaV-3 ecological management objective. [ABSTRACT FROM AUTHOR]