VINEYARD ECOSYSTEMS PROGRAMME

Undervine Ground Cover Habitat for Mealybugs

 

Mealybugs are the main insect that spreads (“vectors”) grapevine leafroll-associated virus 3 (leafroll virus), a damaging disease that negatively affects vine growth, berry size, yield, fruit colour and flavour, and can have a detrimental impact on wine style and vineyard profitability. Successful management of the virus includes controlling mealybug populations.

Mealybugs produce honeydew as a waste by-product, and, where honeydew is plentiful, black sooty mould will grow on grape bunches. When sooty mould is severe, it can result in fruit rejection because of the risk of wine taint.

Recent research has explored whether undervine ground cover could serve as a substitute habitat that helps keep mealybugs off grapevines. The rationale behind the research is twofold:

  1. A vineyard ecosystem that is managed to exclude most plant communities gives rise to a virtual monoculture dominated by the grapevine. Such management often relies heavily on at least 2 herbicide applications to the undervine area, reducing alternative habitat and host plants for mealybugs.
  2. A vineyard where species diversity is promoted is likely to increase interactions between beneficial species and their prey and hosts, with biological control outcomes contributing to a well-balanced, functioning ecosystem.

MOVING AWAY FROM MONOCULTURE

Abundant undervine ground cover, especially that which incorporates a cultivar like white clover (Grasslands Huia), provides an alternative habitat for mealybugs in the vineyard.

In vineyards with few alternative host plants, grapevines are a monoculture, meaning they are essentially the only habitat mealybugs can colonise.

Research results are not yet complete. It seems, however, that maintaining an undervine ground cover provides a long-term, stable, alternative habitat suitable for mealybugs; to date, there has been no evidence of mealybugs on the clover migrating onto the grapevines.

RESEARCH SUMMARY

Longtailed (Pseudococcus longispinus ) and citrophilus (P. calceolariae ) mealybugs are commonly found in New Zealand vineyards. In recent years, researchers have explored options designed to mitigate the risks of mealybugs causing black sooty mould and/or spreading leafroll virus to healthy vines. Studies have tested whether undervine ground cover could serve as a substitute habitat that helps keep mealybugs off grapevines.

Investigations have been undertaken as part of the Vineyard Ecosystems Programme, managed by Bragato Research Institute and co-funded by the Ministry of Business, Innovation and Enterprise. A separate Plant & Food Research study has also focused on managing vineyard ground cover plants, mealybugs and leafroll virus. In all of the trial blocks involved in these studies mealybugs observed on vine leaves were not numerous enough to result in fruit being rejected because of sooty mould.

Results indicate that some undervine ground cover plants do support mealybugs throughout a growing season. In particular, Grasslands Huia white clover – a commercially available cultivar in New Zealand – appears to be readily and preferentially colonised by mealybugs and can be sown to augment existing white clover in ground cover. Other plant species commonly found with mealybugs are hawksbeard, and, to a lesser extent, subterranean clover and dove’s foot.

VINEYARD ECOSYSTEMS

The research conducted in the Vineyard Ecosystems Programme explored the relationships between ground cover, mealybugs and leafroll virus in 8 Hawke’s Bay Merlot study blocks over 3 years. Four of the blocks used “contemporary” management treatments that included the use of insecticide and herbicide, and 4 used “future” management treatments that did not use herbicide – although 1 vineyard in the latter group did use insecticide for mealybug control.

Mealybug habitat preferences were monitored, and alternative host plants, including several commonly found clover species and hawksbeard, were shown to support large numbers of mealybugs for the duration of 3 full growing seasons.

Various monitoring methods (pheromone traps, vine leaf and ground cover plant assessments) confirmed the citrophilus mealybug to be distributed in the trial blocks.

Results showed that, when present, ground cover host plants like white clover and hawksbeard supported large numbers of mealybugs, thereby helping to separate mealybug and grapevine populations.

By contrast, in vineyards with an unstable ground cover habitat (typically, a result of repeated herbicide use), mealybug colonisation of grapevines was often greater relative to vineyards with more ground cover plant diversity. Notably, many of the mealybug-preferred host plants were either absent or rarely found in the contemporary blocks.

STABLE GROUND COVER

The 5-year vineyard ground cover research conducted by Plant & Food Research (concluding in 2021) tested the hypothesis that, when sustained on a stable ground cover habitat containing suitable host plants, mealybugs pose a reduced risk of moving onto and between grapevines and transmitting leafroll virus to healthy grapevines. The trial comprises 21 plots (0.4 hectares each) within a single Merlot vineyard in Hawke’s Bay (19 hectares). Fourteen plots have had Grasslands Huia white clover added to the undervine area (UV-C), and 7 plots comprise an undervine herbicide-treated control (C).

The Grasslands Huia white clover has proven itself to be a mealybug “sink” relative to all other ground cover plants sampled from within the block: mealybugs were associated with 30-50% of the clover samples collected, and this association persisted for the duration of monitoring in 2018 and 2019. To date, however, it has not been shown that the number of mealybugs found on vine leaves has been consistently influenced (reduced) by the addition of clover to the undervine area. Nor has virus incidence been reduced in clover-planted plots relative to control plots.

In herbicide-treated plots the relative absence of ground cover plants meant that their use by mealybugs was also low, with less than 10% of plant samples found with mealybugs.

Various monitoring methods (pheromone traps, vine leaf and ground cover plant assessments) confirmed the citrophilus mealybug to be widely distributed in the plots. Mealybugs were present on vine leaves and on the undervine clover.

On a plot-by-plot basis, mealybug distribution was patchy, with 6 neighbouring plots  showing higher numbers of mealybugs on vine leaves and the same plots revealing the majority of leafroll symptomatic vines. With the exception of those “hotspots”, however, monitoring indicated that mealybugs appear to reside on clover persistently and preferentially (over grapevines).

In spring 2018, canopy development was impaired in some (but not all) of the undervine clover-planted trial plots. As a result, undervine clover plantings were changed to alternating vine-to-vine intervals (a distance of 1.8 metres), meaning that, from autumn 2019, just 50% of the original undervine clover remained.

As part of the ground cover research, a 2018 study explored mealybug preference for 5 clover cultivars under laboratory conditions. The results suggested that subterranean and crimson clover could also provide habitat for mealybugs and that the nectar produced by flowering clover plants in ground cover could attract mealybug parasitoids, potentially increasing parasitism rates among the mealybug populations.

2020 AND BEYOND

Interim results of the stable ground cover study indicate the large numbers of mealybugs present on the clover did not result in an increase in leafroll virus incidence relative to areas without clover.

More insights into these and other aspects of the 5-year study are expected once the 2020 and 2021 data are analysed and the results interpreted.

 

ACKNOWLEDGEMENT

This update was written with the assistance of Vaughn Bell, Senior Scientist, Winegrape Entomology & Virology, The New Zealand Institute for Plant & Food Research Limited. It summarises findings from (1) research commissioned by Bragato Research Institute for the Vineyards Ecosystems Programme, co-funded by the Ministry of Business, Innovation and Employment (MBIE) and (2) research commissioned by Plant & Food Research Ltd.

Additional reading

This article first appeared in the August/September 2020 issue of the New Zealand Winegrower Magazine.