Larry Smith and the Riverside Gardens team talk all things pots, plants and pruning in their weekly gardening column.
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With the warmer weather arriving across Shepparton and the Goulburn Valley, our gardens are putting on heaps of fresh new growth and looking wonderful after the toils of winter.
Unfortunately, warmer weather also signals the return of a couple of pests that have become increasingly familiar to local gardeners.
Elm leaf beetle and citrus gall wasp are two garden pests we should definitely not ignore as they become active in our gardens.
Although they are completely separate pests requiring different approaches to treatment, they are both controlled best by identifying the problem early and treating it at the right time according to their lifecycle.
Shepparton and surrounding towns are fortunate to have some magnificent elm trees.
Apart from their majestic beauty, a mature elm can provide an enormous amount of summer shade, something particularly valuable through our hot summers.
Elm leaf beetle can cause serious damage to these trees. Adult beetles chew distinctive holes in the leaves, while the larvae feed between the veins, leaving foliage with a brown, skeletonised appearance.
Repeated severe infestations can weaken trees over time causing the decline in its health and make it vulnerable to other pests and diseases.
The most important message is don't wait until the tree is brown before deciding to do something.
The elm leaf beetle develops through four life stages over the warmer months going from egg to larva, pupa, and adult.
The stage of their lifecycle and the time of the year determines the best way to treat the problem
Mature beetles that have survived winter in bark crevices, woodpiles, or nearby sheds and buildings will emerge in spring and fly to nearby elm trees and start chewing on the leaves.
This usually occurs in early October, and you may notice the shot-hole damage start to appear on the leaves.
At this stage there are several methods of treatment, and the size of the tree will largely determine which is most practical.
For smaller trees, a foliage spray can be an initial treatment. A Carbaryl Garden Insecticide that lists elm leaf beetle among its uses is one option, however, it is a broad-spectrum insecticide, so care needs to be taken around beneficial insects when spraying.
Spraying a large established elm is another matter altogether.
Achieving good coverage safely becomes increasingly difficult as the tree gets larger and will require the use of systemic insecticides.
These can include soil treatments using products specifically registered for elm leaf beetle control or professional trunk-injection treatments.
By November the main population of beetles will have emerged, and the female will have started to lay eggs which are bright yellow lemon shaped and grouped in a double row on the leaves.
After seven to 10 days, the larvae, that look like very small caterpillars about two millimetres long and black in colour will hatch-out of the eggs.
The emerging larvae will feast on the leaves sometimes skeletonising them to leave just the veins.
As they grow, the larvae will change to yellow and black and start to migrate down the trunk to pupate in the ground or in the textured bark at the base of the tree.
Trunk banding can now be used to break the life cycle of the beetle
This is done by applying two sticky gel bands like ‘Go Natural Tree Guard’ around the trunk at least a hand span apart and about a metre and a half or more above the ground.
This catches the larvae before they have a chance to pupate and interrupts their lifecycle preventing the next generation.
The combination of the two stages of treatment should give you adequate control, but it is worth keeping in mind any tree already struggling with drought, compacted soil or poor growing conditions is in a much poorer position to cope with repeated insect attack.
Like elm leaf beetle, the key to controlling gall wasp is understanding its lifecycle
Citrus gall wasp produces the distinctive swollen, woody lumps we increasingly see on lemons and other citrus. These galls contain developing larvae and can weaken the tree, reduce fruit size and yield and sometimes cause branch dieback.
In our region, adult citrus gall wasps generally emerge from mid-October to mid-December.
After emerging the adult beetle has about a week to mate and lay its eggs beneath the green bark of newer growth.
Traditional control involves pruning out the active galls before the next generation of adults emerges.
Galls become increasingly visible through autumn and winter, making this a good opportunity to control them.
However, heavy pruning at the wrong time can encourage lots of soft new growth which the gall wasp love.
One strategy worth considering in our local conditions is to delay general pruning until late November after the peak gall wasp emergence period has passed.
This can be incorporated with the use of sticky gall wasp traps and, or Kaolin Clay spray to give effective control
Citrus gall wasp and elm leaf beetle can spread from tree to tree, an ignored tree can contribute to problems in surrounding gardens.
So, it is important to identify the pest correctly, understand its lifecycle, treat it at the right time and, just as importantly, don't leave your problem for the neighbour to deal with.
Whether it's a beautiful old elm providing shade on a hot Shepparton afternoon or the backyard lemon tree supplying friends and neighbours, these plants are worth looking after.