Category Archive: Pest & Disease

Citrus leaf miner – silvery lines on leaves.

There is one pest that has been making an appearance in almost every garden, and that is the citrus leaf miner, Phyllocnistis citrella. They are a common pest that attack citrus trees from summer to autumn, especially when the weather isn’t consistent. But they can also prove to be a problem in the early months of spring when the new season’s growth starts to emerge. The adult is a tiny moth that is silver-white in colour with yellow markings and they have an approximate wingspan of 5mm. The adult moth lays their eggs onto the undersides on the new foliage, and once these eggs hatch that is when the symptoms and damage is most noticeable.

When the eggs have hatched the larvae begin to tunnel in between the fine layers of the leaf. This process usually takes the young caterpillar 5 days to do. Affected trees will show symptoms in the foliage and overall appearance. Silver aztec maze designs appear on the leaf surface and can cause the foliage to discolour overtime. Foliage can also be distorted and curled. Other symptoms can cause plants to wilt and in heavy infestations can cause affected trees to suffer from stress and remain unhealthy for long periods of time.

When the affected foliage starts to curl along the margins the larvae are fully grown. The larvae curl the sides of the leaf to create a ‘shield’ to protect itself as it goes through pupation. The pupation can take up to 3 weeks to complete and then the adult wasp emerges and the cycle repeats.

The best method of control is to prune off all affected growth then follow up by spraying an organic pesticide such as EcoPest Oil by Multicrop or Pest Oil by Yates. Don’t apply it on a hot sunny day.

Citrus leaf miner can be found on other plants so make sure to check over surrounding plants. Also you could avoid using a high nitrogen fertiliser in summer, as this promotes very soft new growth that makes them very attractive to these insects.

By Bonnie-Marie Hibbs

Images by Bonnie-Marie Hibbs

Scale insect – dots on a lemon tree.

It is just amazing how many scale insects are on just this one lemon, let alone what must have also been on the branches.

 These scale eat the goodness out of the plant cells. They can literally bite into the skin of the plant.

Heavy infestations like this one will weigh heavily on the plant. Often a weaker plant will then be left vulnerable to a getting a secondary problem. One such secondary problem is sooty mould which will grow on the sugary waste from the scales called honeydew. Ants may also may inhabit the plant in greater numbers and even assist scales by carrying around young insects and clearing away all the sweet sugary honeydew that might otherwise choke a scale colony.

The plant branches and leaves may then have poor distorted growth and eventually branches could even die. Fruit like this can be contaminated or dis-coloured, although this particular fruit still appeared to be ok inside.

Scale are weird because they don’t really look like insects at all. For most of their life cycle they are hiding under protective shells. They overwinter as mature females, and lose the use of their legs.

In spring, the females produce ‘crawlers’ which are very young insects which move out from under the shells and move to new feeding sites. They then form the shell like scale and can no longer move. Many generations can be produced over one summer. They infect fruit like this late in the season. Only the small males fly, but ‘crawlers’ can be blown in the wind.

Control: This plant should have been sprayed with white oil (also sold as Pest oil) in the warmer months, being careful not to burn the plant by doing this on a cloudy day, and maybe even with a test spray. It could still be white oiled in winter, but the insects are much more protected by then and it will be less effective. They have literally pulled down their shutters. You could also prune the tree now and dispose of the branches in the bin. An insecticide like Carbaryl could also be used, but stay on top of this problem with white oil, and instead wait for the ladybirds to help you in summer.

The scale insects will be smothered by the pest oil and die off. They tend to dry up and drop from the tree or can be rubbed/washed off by spraying the tree with water from a hose etc. Sometimes two to three treatments may be required to kill all the scale and baby crawlers and these sprays can be up to a month apart.

Scale under the Gardenworld microscope

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