Sustainable management of adult Costelytra zealandica (Coleoptera: Melolonthinae) damage in Marlborough vineyards

Conventional agriculture is facing many challenges to provide food security for a constantly growing human population. Unfortunately, it still relies heavily on fossil fuel-derived inputs, such as pesticides, which affect human health, biodiversity and contribute to global warming. In New Zealand, the endemic grass grub Costelytra zealandica (Coleoptera: Melolonthinae) has been a pasture pest for more than 100 years. By consuming plant roots, its larvae reduce pasture yield, affecting both dairy and meat production. Recently, adults of this species have been found feeding on horticultural crops, such as kiwifruit, avocado, and vines, amongst others. In vineyards, adults damage vines by feeding on leaves, shoots and inflorescences and can produce a substantial defoliation, reaching in some cases 100%. For this reason, prophylactic use of synthetic pyrethroids is practised as the main approach to control this pest. Pyrethroids are broad-spectrum insecticides, having clear impacts on arthropod diversity, but also their continuous application can have detrimental effects on human health and, like all such chemicals, can lead to pesticide resistance. Therefore, the aim of this work was to understand C. zealandica adult behaviour as it approaches and feeds on vine foliage and thus evaluate a sustainable approach to reduce its damage in vineyards without using pesticides. For this reason, a range of experiments were carried out on vineyards in Marlborough area of New Zealand, in the Awatere Valley (41°44’S; 173°52’E) during 2014 and 2015, and in Blenheim (41°33’S; 173°55’E) during 2015. By studying beetles’ distribution, through adult and larval sampling within vineyard blocks, higher adult and larval numbers were found at the edge of the vineyard compared to its centre. This distributional pattern was probably an expression of a relict adult behaviour in this species, in which females respond to plant silhouettes against the sky to feed and mate. Is therefore likely that in Marlborough vineyard-dominated landscapes, vines were the most abundant plant silhouettes, which might explain adult and larval abundance at the margin of studied vineyard blocks. In another experiment, by removing adults that landed on the vine foliage at several times after their daily flight activity started, it was demonstrated that females land before males on the vine foliage. Literature suggests that C. zealandica females attract males by releasing their pheromone, phenol. Based on those results, different experiments to evaluate the contribution of feeding and landing deterrents to reducing beetle damage were established at the studied vineyards. A novel, naturally-based approach to reduce adult damage in Marlborough vineyards by applying silica-derived feeding deterrents, such as hydrophobic particle films (HPF) and diatomaceous earths (DE) on the vine foliage and secondly, by placing crushed mussel shells (MS) (Perna canaliculus; Mollusca: Mytilidae) under the vine-row as mulch was investigated in this work. A significant reduction in vine damage was produced by HPF and DE compared to control, with a 46% damage reduction in Pinot Noir variety. MS significantly reduced the number of adults that landed on the vine plants when compared to control in Pinot Noir. To explore potential synergies between HPF and MS, these were combined in another experiment during the 2015 adult flight season at Blenheim. Adult C. zealandica damage was significantly reduced by 33 and 73% by HPF and MS, respectively. Using infra-red sensitive video cameras it was demonstrated that MS significantly reduced activity of flying adults above treated vine plants (cv. Pinot Noir), which led to a 28% increase in grape yield. It was suggested that the light reflective properties of MS reduced the necessary plant contrast with the sky, altering adult landing dynamics and subsequently, vine damage. These findings contribute to the reduction of pesticide use within this agro-ecosystem, and also remove the need for disposal of large quantities of these shells, which would otherwise go to the local landfill. There, the proteinaceous parts of the shell waste generate methane, a key contributor to greenhouse gases due to the anaerobic conditions present in the landfill. In addition, this management is in agreement with the cultural perception of kaitiakitanga, which is the spiritual consideration of protecting the land for descendants, rooted in Maori (New Zealand’s indigenous people) cultural heritage. In this work, a sustainable pest management strategy to reduce C. zealandica damage in vineyards was proposed, highlighting the importance of understanding insect behaviour to reduce pesticide applications and thus, promote environmentally-sound agricultural systems.