what a pest

HELP! WHAT'S WRONG WITH MY PLANTS?

Words Kath Gadd, Mallee Design malleedesign.com.au

Well, it would seem La Niña is here with a vengeance, resulting in way too much rain and very uncomfortable humidity. It has been a tough summer for many plants. Not only have the weeds enjoyed an extreme growth spurt but the bugs and insects are out in full force too, and the extra moisture in the air is also great for moulds and fungus to flourish. There are four factors that may be challenging your plants' health at the moment. Here’s a quick run-down on what they are and how to treat them…

Sap suckers

Sap-sucking insects, such as scale, aphids and mealybugs, feed on the sugary-liquid that travels through plant tissues, slurping up the nutritious solution that should be feeding the plant! They tend to multiply quickly, weakening plants and often deforming foliage. Many have tiny waxy bodies and secrete honeydew as a by-product of feeding. The honeydew attracts fungal diseases like sooty mould and also ants, which may protect the sap-suckers from predators and carry them to new plants.

Because they can be so prolific and so tiny, the best way to treat them is with an insect spray. A white oil treatment is effective for many sap suckers, it coats the breathing pores of insects, suffocating them, and can be easily made up at home with dishwashing liquid and oil (you can also use other homemade mixtures like a chilli or garlic oil). Shop-bought pest oils also work – they usually operate on the same principle of suffocating pests and may contain other compounds to ensure knock-out. Pyrethrum, another good option, is slightly different. The active compound is naturally-derived from the pyrethrum daisy and works by inhibiting an insect’s nervous system.

Whatever spray you’re using, try to spray only the affected areas on both sides of the leaves/branches. Chemicals like pyrethrum are non-specific so will also kill the good insects and bees around your plants if you use them too much. Another option is manual removal. Some people have removed sap suckers by patiently scraping the leaves, or if the infestation is too large, blasting them off with a pressure hose.

Leaf chewers

Leaf-chewing insects have specialised mouthparts and often target young, tastier leaves and little seedlings. They include things like caterpillars, sawfly larvae, white fly, snails and slugs. They’ll often secrete toxins that make them unpalatable to potential predators like birds, and in that way, continue feeding on plants unharmed.

Some of the pest oils/sprays work for leaf chewers but you can also try manual strategies, depending on the insect and its life cycle. For example, the caterpillar on this white cedar (pictured left) has been absolutely decimating the foliage. The grubs eat the leaves during the day then fall to the ground at night. Putting up a foil barrier around the base of the tree’s trunk can stop them being able to climb back up in the morning.

In the case of sawfly larvae, pruning off affected foliage before the infestation gets too large may be enough. Sawfly larvae feed mostly on bottlebrush. If found on established, adult plants they shouldn't be a problem but on vulnerable, young plants you need to remove them before it’s too late.

Deficiencies

Deficiencies usually present as discolouration in the leaves and are more common in rainy weather. Heavy rain washes out nutrients in poorly soils – the washing out of nitrogen (N) and iron (Fe) can be a particular issue for natives.

Yellowing of the leaves is one of the most common signs of deficiency. It can be hard to tell exactly what a plant is deficient in but providing some slow-release fertiliser, iron chelates and/or adding trace elements like manganese, zinc and boron can help. Trace elements play a role in unlocking nutrients to plants, often via soil microorganisms. Soil microorganisms are the main underground energy hubs that convert inorganic forms of nutrients like nitrogen into bioavailable forms that plants can use.

Moulds and fungus

Mould and fungus can spread to plants via wind, water, soil and animals. Sooty mould is one of the most common variants on garden plants and it settles on the honeydew secreted by sap-suckers. It doesn’t directly damage the plant itself but when it smothers lots of leaves, it can stop the plant from being able to photosynthesise. The best way to treat the mould is by treating the source of the problem, the sap-sucker! But you can also manually wipe off thickly coated mould with a damp cloth.

Other funguses and moulds to look out for are myrtle rust, which is detrimental to many of our native trees and shrubs, beginning as purple spots and turning into yellow pustules on the under-side of the leaves, this fungus needs to be removed immediately by cutting off affected leaves and branches and disposing of them appropriately, so the spores don’t spread.

Phytophora is another fungal mould that appears in humid, wet weather – it can appear as dieback on the leaf, turning into blight or rust, but also as root rot on larger specimens. It is caused by plants not coping with poor drainage and may begin as a problem around the root zone and not show up on the leaves until it is too late.

Fungicides have limited scope and should not be expected to cure heavily infected plants, it is better to take a note of what is suffering and try to improve drainage and air flow around the plant.

The most important thing to do is to keep an eye on your plants by checking them closely and regularly, especially in this weather when pests and diseases are rife.