Persistent problems with infestations of thrips close to harvest, in Mpumalanga and Limpopo. By Elleunorah Allsopp (ARC Infruitec-Nietvoorbij) and Michael Stiller (ARC-PHP Biosystematics)
Researchers at the ARC, funded by the ARC and SATI, investigated reports by table-grape growers in Mpumalanga and Limpopo of persistent problems with infestations of thrips close to harvest that cause feeding damage to secondary shoots and leaves.
Thrips were monitored over two seasons, with blue and yellow sticky traps in four vineyards adjacent to citrus orchards, and one vineyard with no citrus nearby in the Marble Hall and Groblersdal districts. Results over two seasons confirmed that flower specialists like western flower thrips and blossom or "kromnek" thrips (Frankliniella species) were the most abundant thrips during flowering.
No halo spot damage due to egg-laying, or corky scarring due to thrips feeding on very young berries were observed. Monitoring and early season sprays were effective in preventing damage by the flower specialist thrips.
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When the grapes started ripening and the secondary growth flush began, citrus thrips (Scirtothrips aurantii) was the most abundant species in all of the vineyards. Feeding damage to young shoots and leaves was evident (Figure 1). Feeding damage on berries occurred occasionally on mostly late ripening cultivars when thrips numbers were very high (Figure 2), but these thrips are highly mobile and fly or jump away as soon as a bunch is disturbed. They prefer to hide away in the unfolding young leaves where they are protected from potential predators. Severe shoot damage can result in the cessation of shoot growth and shoot die-back. This compromises the accumulation of sufficient reserves postharvest, to initiate budding in the next growing season after winter dormancy, ultimately resulting in reduced production. This phenomenon of citrus thrips feeding damage to secondary shoot growth and berry scarring close to harvest has also become a regular occurrence in certain areas in the Western Cape, namely the Hex River Valley, Piketberg-Porterville region, and the Olifants River Valley.
Why late season thrips are problematic
Various factors contribute to the late season explosion of citrus thrips populations on grapevines. These thrips occur on many indigenous and other cultivated plants in and around vineyards. This means that growers cannot eliminate them from vineyards with early season sprays, as more thrips simply move back in when chemicals are no longer active. Citrus thrips prefer new growth with high nutrient quality and as the surrounding natural vegetation dries out during summer, the thrips migrate to the more attractive grapevines. Our research also confirmed that citrus thrips move readily between citrus and grapevines, since their growth flushes do not occur at the same time.
Irrigation and fertilisation of grapevines and citrus increase the nutrient quality of the plants, which massively increases thrips reproduction. The late season influx of citrus thrips on the secondary growth of grapevines often occurs too close to harvest to apply chemical control, due to residue concerns and market restrictions, which means that feeding damage on berries cannot be prevented. Furthermore, grapevine shoot tips and young leaves are covered with water-repellent hairs that reduce the ability of insecticides to penetrate and reach the thrips.
Managing late season thrips
Chemical control aims to protect the susceptible stages of the grapevines (flowering bunches, young berries, and new growth) from thrips. Diligent monitoring to spot the influx of citrus thrips early can enable growers to apply a suitable insecticide to prevent berry damage without incurring residue problems. This is becoming challenging, however, due to tightening market restrictions on the types and number of active ingredients permitted pre-harvest, as well as resistance management and residue concerns.
If the influx occurs too close to harvest to spray – often the case with late ripening cultivars – chemical control should be applied as soon as possible after harvest to protect the secondary shoots and growth points to ensure sufficient accumulation of reserves and ripening of canes.
Biological products like entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN) that attack and kill insects specifically, are ideally suited to manage thrips infestations close to harvest when chemical control is not an option. Research over the last 15 years has already identified several local EPF and EPN isolates that are effective against a variety of insect pests, including western flower thrips and citrus thrips, in laboratory and semi-field trials. This is also driving fruit industry efforts towards commercialising local EPF and
EPN products.
