Their impact can be seen widely in New Zealand and is particularly noticeable on rhododendron, viburnum and photinia by the silver-coloured foliage. This effect is caused by thrips feeding on the plant’s leaf cells and sucking out the contents which leads to the silvering effect. New foliage has higher concentrations of a natural defence compound — grayanotoxin — and emerges healthy and green but will succumb to the same fate.
If left untreated, affected plants can decline and over time can be fatal. Heavy infestations can look unsightly and, if left, can act as host plants allowing thrips to spread easily to susceptible plants elsewhere.
Using an insecticide or insecticidal soap is one option for treating affected plants as are natural biological control methods. Another option and complementary to these is focusing on improving plant health. A healthy plant is likely to have much better resistance. Enhancing plant health can be achieved through improvements to the soil, drainage, use of mulches, particularly in drier areas to help conserve moisture levels. For rhododendrons, for example, applications of acidic fertilisers can be used.
Among the large variety of rhododendrons available, some have more resistance to pests and disease than others. Rhododendrons that have soft and hairy indumentum on the underside of their leaves are less susceptible to thrip damage. Others are naturally more resistant in other ways giving some added protection. Susceptibility to such pests is becoming an important consideration when selecting new plants and thinking how they will perform in the future with our changing climate.
Garden Life is produced by Dunedin Botanic Garden. For further information contact Stephanie Sinton or join a free tour of the Rhododendron Dell on Sunday either at 10am or at 5.30pm — meet in the upper garden carpark.
