Sap-suckers & sooty mould

Sooty mould is not a plant disease as such but various species of fungi, which grow on the sticky honeydew excreted by many species of sap-sucking insects. A very thick mould layer on leaves may cause a reduction in photosynthesis in plants, which can result in leaves falling prematurely. The main concern to gardeners (apart from the sap-sucking pests which caused it) is that a thick covering of sooty mould on a plant’s leaves is downright ugly.

Sooty mould is a serious problem for the horticultural industry. Ugly plants are unsaleable. Similarly in the citrus industry, the main economic damage caused by mealybugs is by the downgrading of fruit quality due to sooty mould growing on mealybug honeydew. The picture below shows a leaf infested by soft scale insects and the sooty mould they cause – pretty ugly eh?

Sooty mold

Insects which produce honeydew include planthoppers, leafhoppers, treehoppers, psyllids, whiteflies, aphids, scale insects and mealybugs – all ‘true bugs’ in the insect Order Hemiptera. These bugs consume vast quantities of watery sap from which they extract the nutrients they require, particularly nutrients containing nitrogen. Many kinds of sugars are also present in plant sap in abundance, which are not required by the insects and are excreted in solution as honeydew. Almost all the dry weight of honeydew is in the form of sugars. It’s worth tasting honeydew to experience just how sugary it is.

Many bugs propel the honeydew away from them so they don’t become trapped in the sticky stuff as it dries. The honeydew falls to foliage below, often in vast amounts when the insect colony is large. Wherever the honey dew falls sooty moulds may grow. The hibiscus leaf covered in sooty mould pictured below was directly below a leaf (underside) covered in greenhouse whitefly (Trialeurodes vaporariorum).

hibiscus sooty mold

But not all bugs throw their honeydew away. Some psyllids allow their honeydew to harden into protective ‘lerps’ (you can read more about them here) while other sap-sucking insects use honeydew to attract ants which act as their bodyguards. Such interaction between bugs and ants is known as trophobiosis and is based on reward (sweet honeydew) for services rendered (protection from predators). The relationship can be specific and involve a single ant species and a single bug species, or one ant species may interact with several species of bugs, or vice versa.

You don’t have to look far to find ants protecting honeydew producing bugs such as aphids, planthoppers and scale insects  – almost any park, garden, orchard or nursery will be harbouring such a relationship somewhere. The picture below shows some ants attending soft brown scale, and you can even see a drop of honey dew.

Anst feeding on honeydew

If ants are efficient at protecting the pest insects from predation such that pest numbers increase, ant control might be something you consider. For example, organic growers of citrus and other fruit have to use ant control as a means of controlling scale insects and other pests on their trees. To put it another way, controlling the ants gives predators and parasites a chance to control the pest insects. Ant control is simple – you can prevent the ants climbing a tree by banding the tree trunk with horticultural glue or some other sticky substance.

There are no fungicides registered against sooty mould – you must control the insect pests which produce the sticky honeydew on which the fungus grows. The fungus will stop growing once the honeydew source is removed, and will eventually dry out and fall off leaves, or it can be hosed off.

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What’s good about ants? Plenty.

Many people, especially gardeners, only see ants as pests, but they couldn’t be further from the truth. These days I work weekends in a garden centre, and when summer arrived every second customer I encountered wanted a product to kill ants. One person even asked me for the “Agent Orange” of ant insecticides! They were somewhat bemused when I asked “What sort of ants are they, and why do you want to kill them?”

ponerine ant
This harmless ponerine ant is holding a water drop in its mandibles.

There are more than 1,300 described species of ants in Australia, but only a minority are considered to be pests. Most ant species are beneficial in some way. Ants are extremely important in the environment and are sometimes referred to as ‘ecosystem engineers’. For example, in bushland, ants are vital dispersers of seeds, a mutually beneficial practice known as myrmecochory. Many Australian plants, about 1500 species or so, have evolved to produce seeds with elaiosomes, which are expendable seed parts containing oil, protein, starch, sugar and vitamins. Ants collect the seeds, and carry them off to their nests underground. However, they only eat the elaiosomes and discard the rest of the seed in a ‘rubbish tip’ section of the nest, or hide them under leaf litter outside the nest. The seeds are then sort of planted, safe from predation or fire, where they can germinate safely.

Mind-boggle alert! Some stick insects exploit these ants’ seed-burying behaviour by laying eggs that look like seeds (see below), and even have an expendable fatty part for the ants to feed on. The eggs are collected by ants and taken into their nests where they are safe from predators. When the young stick insects (nymphs) hatch they even look and smell like ants. The nymphs then wander out of the nest, past the unsuspecting ants, and up the nearest gum tree to feed on the leaves.

Phasmid eggs

Anyway back to the ants. Ground-dwelling ants aerate the soil while digging their nests, which allows water to penetrate the soil more effectively. Those ants that make their nests in dead wood aid the decomposition process of that wood. Many ant species are predators or scavengers consuming vast numbers of the eggs, larvae and adults of insects and other invertebrates − either dead or alive. Ants in turn are food for many creatures such as birds, echidnas, reptiles and other invertebrates.

Some common ant species that are predators and scavengers include funnel ants (Aphaenogaster spp.), green tree ants (Oecophylla smaragdina), meat ants (Iridomyrmex spp.) and sugar ants (Camponotus spp. – pictured below). These ants should be encouraged and respected rather than bombed with insecticides.

camponotus

Where I live in Western Victoria there is really only one ant species to be wary of and that is the Jumping Jack ant (Myrmecia pilosula) pictured below – aka Jack jumper, hopper ant, skipper ant. Jumping Jack ant stings can cause anaphylaxis, a life-threatening condition in some people typified by constriction of the throat and difficulty in breathing. Milder symptoms include swelling and itchiness around the wound. They are extremely aggressive ants by nature and move in a characteristically jerky, jumping manner. Their movement may look a little comical, but there is nothing funny about the sting!

jack jumper ant

In a future blog post I will be writing about how some butterflies have interesting associations with certain ant species. In some cases the butterflies are totally reliant on the ants for their survival.

 

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Where do insects go in winter?

Insects are ectothermic meaning they don’t generate their own body heat, and rely on external sources to regulate their body temperature. Freezing temperatures are life threatening to many insects, so how do they survive winter?

One July morning I went down to my vegetable garden to see how the plants had survived the deep freeze (-3°C) of the night before. I wasn’t surprised to find my brassicas were covered in ice crystals, but I didn’t expect to find a cabbage white butterfly (Pieris rapae) larva also encrusted in ice. I decided to keep an eye on it, so after the day warmed up a bit I revisited the garden to find the caterpillar happily munching on the plant. How did it do it?

Cabbage white larva Continue reading Where do insects go in winter?

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Stingers

Jack Jumper ant
Ouch!! I felt a sharp stabbing pain on the webbing between two fingers of my left hand. I looked down to see a Jack Jumper ant gripping my skin tightly with its mandibles and burying its sting in my flesh. I had been winding a hose back on its reel, the ant had taken a ride on the hose, became caught in my hand and went into attack mode. Ever been stabbed by hot needles? That’s what it feels like. Continue reading Stingers

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