The basics of integrated thrips management in blueberries
Thrips constitute a group of about 7 700 species, of which only a few are economically significant agricultural pests. Western flower thrips (Frankliniella occidentalis) and South African citrus thrips (Scirtothrips aurantii) are the most common problem species in blueberries. Onion thrips (Thrips tabaci) are less often encountered.
Adult thrips can fly, albeit weakly, and are also wind-dispersed. Female thrips insert their eggs just below the surface of young leaves, flower parts, and small fruitlets. The larvae are wingless miniatures of the adults. Larvae undergo two instars before entering a non-feeding prepupal stage, followed by pupation in the soil or on the host plant.
Damage
Larval and adult thrips extract plant juices with their piercing-sucking mouthparts, destroying cells and disrupting tissue development. Young plant organs are most susceptible to damage, so the high-risk periods in blueberries are growth flushes and flowering.
After summer pruning, heavy thrips infestations of the flushing foliage deform and stunt growth, reduce photosynthesis, and decrease yields. Scirtothrips aurantii is the main culprit, as it prefers growth tips and young shoots and leaves.
In contrast, F. occidentalis is a specialised flower feeder and usually responsible for egg-laying and feeding damage during flowering and fruit set. Egg-laying stimulates small, raised, corky lesions on fruit, while feeding can cause fruit deformation or scarring.
Thrips dislike cold, wet weather, so fruit damage tends to follow warmer, drier winters and springs in the Western Cape. In the summer-rainfall areas, thrips infestations generally occur earlier, unless cold snaps during flowering suppress pest populations.
Monitoring
Monitoring is based on traps and scouting. Growers can hang yellow or blue sticky traps above the plants in sunlight. Traps must be in place after summer pruning, before the growth flush, and before flowering.
Western flower thrips (but not other species) can be trapped more efficiently by placing a lure on the sticky traps. The lure attracts males and females. However, once the thrips have located a suitable host, they seek shelter and seldom fly far again. This limits their exposure to lures and the likelihood of catching them on sticky traps.
When scouting, tap leaves and flowers over a white surface to dislodge thrips. Note that this method may miss adults, which can fly away when disturbed.
Comparing different monitoring techniques is one aim of a new BerriesZA-funded project, discussed elsewhere in this newsletter.
Chemical control
There are no thresholds for thrips. Given the pests’ rapid reproduction (they can complete their life cycle in as little as two weeks) and the high damage potential of even small numbers, control is typically initiated as soon as the insects are detected.
In view of the scant chemical control options, avoid calendar sprays. Save pesticides for the critical risk periods and do not spray unless the presence of thrips is confirmed.
Acetamiprid is registered for thrips control on blueberries. Due to its long withholding period (35 days), it’s usually reserved for knocking down thrips on the growth flush after pruning. Two applications are permitted.
Spinosad and spinetoram are also registered for thrips on blueberries. Their shorter withholding periods (3 and 7 days, respectively) allow them to be used during flowering. Both are toxic to bees when directly applied.
Keep in mind that spinosad and spinetoram are in the same insecticide group (spinosyns).
When growers have exhausted the permitted applications for the above active ingredients, they have a few softer registered options. These are best positioned during lower-risk periods.
Some people believe that feeding attractants based on sugar or flower volatiles will improve the efficacy of chemical control. However, there is insufficient research to support their use, and no feeding attractants are registered for thrips in blueberries.
One reason why feeding attractants may be less applicable to thrips than, for example, fruit flies is that thrips respond to several general feeding cues, including plant shape and colour, and switch to olfactory cues only when they’re close to or in contact with the host.
Biological and cultural control
Natural enemies, including predatory mites (such as Neoseiulus cucumeris, Transeius montdorensis, and Amblydromalus limonicus) and lacewings, may reduce thrips during the season when pesticides are not an option. Entomopathogenic fungi could also contribute to thrips control.
Cover crops, weeds, and thrips have a complicated relationship. Alternative hosts can keep thrips out of blueberry plants or serve as breeding grounds for the sucking pests. If an orchard has weeds or cover crops, disturbing them during high-risk periods is likely to send thrips into the blueberries.
Flowering cover crops, particularly clovers and vetches, boost populations of F. occidentalis, creating a risk of spill-over to blueberries.
Establishing alternative hosts around blueberry orchards as barrier or trap crops may assist in controlling pests if pesticides are applied to the alternative hosts, but this will kill beneficial insects in the trap crop.
Growers who don’t want to spray plants that could serve as a habitat for beneficial insects must monitor those areas for thrips to ensure pest populations don’t explode and spill over into the orchard.
Observations suggest that blueberry cultivars differ in their attractiveness to thrips, so replacing highly susceptible cultivars could help to reduce losses on problem sites.
Investigating the underlying reasons for cultivar differences is another goal of the new BerriesZA-funded project discussed elsewhere in this newsletter. The project also examines the association between neighbouring crops and thrips abundance in blueberries.
Acknowledgements
Dr Elleunorah Allsopp. Consultant and researcher.
Desi Fourie. Technical adviser. Viking.
Wilma van der Westhuizen. Technical manager. Nexus.
Image courtesy of Wilma van der Westhuizen.
