I’ve had several enquiries in the garden centre recently about infections to eucalypti, and particularly e. viminalis, which is the type most commonly grown here. Sample leaves show rust-red spots and, often, hard sugar-like lumps. The villain is the lerp psyllid, more pronounceably known as the jumping plant lice. Psyllids belong to one of the oldest insect groupings known to us, including other favourites of ours such as aphids, white fly and scale insects. How we love them all!
Psyllids are small insects that suck sap and, like their host trees, are native to Australia. It is one of the most devastating pests in Australia and there are some 330 different species attacking almost exclusively eucalypti, acacias and melaleucas. Since 2007 it has been noted that at least one of their family has taken up residence in Spain. Believed to be glycapsis brumblecombei, its primary food source, in Australia, is the red gum or eucalyptus camaldulensis but it is also seen on e. delversicolor, e. globulus, e. rostrata, e. sideroxylon and e. viminalis – and the list keeps on growing!
Australia is home to some 900 species of eucalypti; it’s their principal treeand host to a whole range of pests. But the red gum is the most commercially important to Australia; its timber is insect and decay resistant and it is used for fence posts, railway sleepers and furniture. It also, usefully, grows in soils with a high saline level.
Fully grown psyllids resemble very miniature cicadas, those droning insects of high summer. The female psyllid lays between 100 and 700 eggs in one batch. After a 15 day incubation period, the eggs hatch and the nymphs, anchoring themselves to the stomata of a leaf, start to suck the sap and nutrients from the leaves, leaving them washed-out and pale looking. They construct a ‘lerp’ made from starchy material extracted from the host plant. This hard sugary shield protects the nymphs from predators. The name ‘lerp’ is from an aboriginal word meaning house – the perfect fairytale candy house! The nymph will moult five times before reaching adulthood. The 5 mm long yellowish-coloured psyllids are strong fliers and jumpers. They can produce three to five generations yearly, depending on ambient factors: in summer, with ideal temperatures, there may only be eight weeks between egg laying and adulthood. The sucking insects exude vast amounts of honeydew and the foliage is subsequently attacked by sooty mould which sticks to the honeydew coated leaves, turning them black and ugly. Heavily attacked eucalypti suffer defoliation, particularly in summer when the leaves will also dessicate. By the end of summer, the trees can often be looking fairly bare, especially the lower branches.
Monocultures, or large groupings of the trees, are particularly vulnerable and warmer temperatures boost the generation turnover of the insect. The vigour and growth rate of the tree is severely affected. However, eucalypti are extremely tough and tree mortality is fairly rare, except where trees are continuously attacked over several years and/or are weakened by severe drought, fire etc. A slow decline in the health of the tree can, however, leave it open to attack by a range of secondary diseases, some of which may lead to death.
So, that’s the doom and gloom, and here are the answers! Firstly, the insects will not be very active in winter and, as long as the infestation is not too severe, it can be simply monitored. If defoliation occurs, burn the affected leaves and wait on a new flush.
Natural Predators and Biological Control: Ladybirds, some spiders, preying mantis, some birds and a parasitic wasp called psyllaephaegus bliteus. Spraying the canopy with neem oil will clear the insect. If the canopy is too high to reach, then watering a neem oil solution into the root system to be absorbed will have the same effect though more of the product is consumed.
Chemical Control: Bayer Chemicals produce a product called Imicide 3ml which contains imidacloprid. If sprayed over the canopy it will provide good protection and is effective if applied at the first sign of infestation: severely stressed trees may not respond. Fertilise the trees before treatment as it seems to work best on vital trees. If the canopy cannot be reached, then the treatment has to be applied professionally by injection. Two injections will be necessary, at two month intervals, and that will usually provide protection for a year.
Lots of people object to eucalypti, saying that they are an invasive and alien species – well, that’s true, but then so are many of our commonly- used garden plants! They are stately, beautiful trees with their grey-blonde peeling bark. It would be sad to lose these ghostly giants.
Warning! Onanother topic,I’ve been asked to remind all readers that the sap of fig trees can be highly dangerous, provoking burns and nasty sores on the skin. Always wear protective clothing and goggles when pruning the trees. I’ll cover this in more detail in a future article.
Lorraine Cavanagh has lived in Spain for 23 years; a mother, grandmother and hispanista, her passions are plants, the environment, food and drink, and travelling within Spain. A landscape gardener and writer, she’s always happy to give advice. Call and see her at Viveros Florena, near Cómpeta – have free coffee and cake in their tea-rooms.
Hours: Summer 9 – 2 (July, August and September)
Winter 10 – 4
Her book Lorraine Cavanagh’s Mediterranean Garden Plantshas been nicknamed ‘the bible’. The new edition at €24.90 is now generally available throughout Spain.