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Bug Detection

They bite, suck, even try to rasp your plants to death. How to work out which bug is doing what.


With Ruud Kleinpaste

Being a plant doctor is not easy. In our gardens we're confronted with a whole range of damage patterns on our plants. There's holes in leaves, there's grey leaves, there's sooty mould and honeydew. And who does what? Forget about the classification of insects. I divide my pests into three categories; the chewers, the suckers and the raspers. And it all depends on what their mouthparts look like.


A chewer makes large holes in leaves. It takes plant material away, and that's the key to chewing insects. They remove leaf material and digest it.

Shot holes on camellia On this camellia bush we have very specific chewing damage. It's called 'shot holes' because there's tiny holes on the leaf. It's caused by bronze beetle in springtime, but this damage was caused almost a year ago. You can see that because the edges are totally brown, and that means that it's old damage.

The question is, what do chewers actually do to the health of the plant? What are the consequences?

First of all, they are just like my pair of secateurs — selective pruners, and they take away bits of green material. Any plant that is well-established — think of your roses — will grow back copious amounts of leaves after pruning. We take 80% of our roses off in winter, so why wouldn't a plant be able to stand that? It will, don't make any mistakes. But the thing is, selective pruning can sometimes disfigure a plant a little bit and you might need some control.

Leaf miner damageChewers like caterpillars, beetles, weevils, katydids and earwigs can be easily controlled with chemicals like Maldison, Carbaryl and Mavrik or, organically, you can control them with Pyrethrum or Derris Dust.

For slugs and snails, you can use snail bait, or what about beer traps or hand-picking?

Leaf miners are protected inside the leaf and therefore need systemic insecticides, such as Orthene or Target.


Think of a digger. The way it drills into the soil is similar to the way suckers drill into plants. If the digger were to hit a water main, it would be exactly the way the sucking insects find their food. Suckers stick their mouthparts into the phloem of the plant and extract the sweet plant sap within.

Sooty mouldSuckers excrete most of that sweet stuff as honeydew, which is eagerly harvested by ants. If you see a heap of ants going all over your plants, you know that a sucker must be at work, such as aphids on the underside of the leaves.

Excess honeydew is a brilliant breeding ground for sooty mould. Here's a good infestation of mealy bug, causing sooty mould all over the place, including the bricks.

Scale insects on flaxThis is a good example of massive numbers of suckers hammering the flax. They are scale insects and they are nicking all the sweet sap, causing local deficiencies in the leaves (discolouring of leaves). You see, that sweet sap is normally used for plant growth, including leaves, and the flax is suffering.

Most suckers like aphids, scale insects, mealy bug, passionvine hopper, whitefly and green vegetable bug are like flying hypodermic needles, and that means they can spread diseases, especially viruses. That can be very serious for the health of the plant.

General chemical control of sucking insects would be systemic insecticides such as Orthene or Target. With scale insects and mealy bugs, use mineral oil or mineral oil mixed with insecticide. Organic control of most suckers can be achieved with oil, fatty acids or Pyrethrum.


Silvering effect caused by thripsBack to the digger again. The way its scraping blade works the soil surface resembles a rasping creepy-crawly attacking the outer layer of the cells on the leaf.

Raspers are really minute invertebrates with mouthparts not unlike a scalpel blade, or buck teeth. They scrape the top layer, or epidermal cells, of leaves and lap up their contents. The cells then fill with air and that causes a discoloration — silvering, bronzing or yellow stippling. In this case, this rhododendron has got silvering leaves caused by thrips.

Thrips on azalea tend to cause a distinct bronzing effect. The thrips happily live on the underside of the leaves, as long as it's out of the sun.

Two-spotted spider mites are also raspers. They love to damage cells in patches, causing yellow stippling. Spider mites usually operate on the underside of the leaf and love to surround themselves with lots of fine silk webbing.

High population densities of thrips and mites can cause a real mess. More often than not, they can debilitate their host plant as well, so control is usually needed.

Bronzing effect caused by thripsThrips are insects and will be killed by insecticides such as Maldison, Mavrik or Orthene or, organically, by fatty acids or Neem oil.

Mites are not insects and regular insecticides often don't work on them. You'll need a miticide, such as Dicofol or, organically, use fatty acids, mineral oils or simply mist the leaves with water.

So, next time you see some leaf damage on your plant, try to classify it as either a chewer, a sucker or a rasper, because if you know who did it, it is easy to get the right control method.

Reproduced with permission from NZOOM Home and Garden content,
from the previous website of  TVNZ News

The views expressed here are not necessarily those of the RNZIH

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Last updated: March 1, 2021