Thrips obscuratus in New Zealand vineyards : it's biology and effects on Botrytis cinerea infection

Abstract

This research project investigated aspects of Thrips obscuratus biology and its interaction with grapevines and botrytis bunch rot in two New Zealand vineyards. Flight activity, phenology and short-distance movement of T. obscuratus were assessed in the field and the influences of the fungus and the insect on B. Cinerea incidence and severity, skin scarring and phenolics and grape yield were investigated in greenhouse and field experiments.

The numbers of airborne adult thrips were monitored at Canterbury House Vineyard (now Waipara Hills Vineyard), Waipara, and at Neudorf Vineyard, Nelson, from spring to autumn in the 2003/04 and 2004/05 season. Thrips numbers were highest during the grapevine flowering period, being mostly T. obscuratus (90%), and higher at Neudorf Vineyard than at Canterbury House Vineyard. In the latter vineyard, thrips numbers per inflorescence at full bloom were higher in the later flowering Sauvignon Blanc than in Riesling.

T. obscuratus numbers at different life-cycle stages were also monitored in the two vineyards, from two weeks before full bloom amongst Sauvignon Blanc and Riesling varieties. Flowers from grapevines in both regions were stained to show thrips eggs, which were commonly observed in the pedicels of flowers. Only small numbers of larvae were found in the traps underneath the vine canopy and no emerging adults were caught in the emergence traps placed on the ground, indicating that little reproduction had occurred in the vineyards.

At Canterbury House Vineyard in spring 2005, T. obscuratus movement from three flowering inter-row plant species, buckwheat (Fagopyrum esculentum), phacelia (Phacelia tanacetifolia) and alyssum (Lobularia maritima), to grape flowers was investigated to determine whether such floral resources could be alternative hosts of T. obscuratus, or act as trap plants. Thrips on these plants were marked with a foliar spray of 2000 ppm rubidium chloride once during full bloom of the grapevines. Numbers of marked T. obscuratus adults collected from grape flowers on adjacent vines were very low for all inter-row species, indicating little movement between them.

An investigation into the potential role of T. obscuratus in vectoring B. cinerea conidia to grapevine flowers showed that 20 thrips exposed to sporulating B. cinerea deposited 800 viable conidia per inflorescence and caused 88% flower rot. Scanning electron microscopy of T. obscuratus revealed that many conidia were distributed over the cuticle of the insect, most being trapped between the cilia on the wings or attached to the numerous setae of the body.

The behaviour of T. obscuratus on grape flowers, observed in the laboratory by continuous video recording over 10 minute periods, showed that T. obscuratus spent most time walking and feeding on nectar, pollen and stigma tissues. The grooming behaviour between feeding and walking events and the feeding positions were considered sufficient for it to deposit B. cinerea conidia on the naturally susceptible and wounded areas of the flowers.

Greenhouse experiments conducted in 2004/05 and 2005/06 used Mullins' plants of the white varieties Riesling and Sauvignon Blanc in both seasons and Pinot Noir in the first season only. Silk bags enclosed 10 T. obscuratus collected from the field and conidia of the pathogenic B. cinerea marker strain 879-1 nit1 around each inflorescence for 7 days. In 2004/05, Pinot Noir berries had much lower incidence of B. cinerea infection than did the white varieties, whose overall incidence increased from fruit set to harvest in both seasons. Presence of T. obscuratus increased infection of grape inflorescences of both white varieties in both seasons but not of Pinot Noir. The presence of T. obscuratus and B. cinerea at flowering also caused significantly lower bunch weights in some treatments, lower numbers of berries per bunch and higher incidences and severity of B. cinerea, with Riesling being most susceptible. Field experiments conducted at Canterbury House Vineyard and Neudorf Vineyard in 2004/05 and 2005/06 with the same treatments demonstrated similar effects on infection incidence, severity and bunch sizes.

In 2005/06, assessment of grape bunch scarring by T. obscuratus and B. cinerea in greenhouse and field experiments showed increased scarring for some B. cinerea treatments and more consistent increases with T. obscuratus treatments in both varieties. Analysis of total phenolics in berry skins of Riesling reflected the trends in scarring, being higher in the B. cinerea and T. obscuratus treatments than in the control. When one application of the insecticide pyrethrum was made in the field at full bloom, there were fewer B. cinerea infections higher yields and lower percentages of scars than when flowers were untreated.

An identification method was developed for T. obscuratus by DNA sequencing of the 'Folmer' fragment of the mitochondrial gene COI. DNA sequences of larvae and adults were established that could allow them to be distinguished from other thrips, which could formerly be done only for the adults using traditional morphological identification methods.

This study has provided information about the interaction between T. obscuratus and B. cinerea infection pathways in grapevines under laboratory, greenhouse and field conditions, which may improve the developments of control strategies in the future.