Denis CrawfordWhy do we have plant pests?

Around about now your garden should be teeming with insects. Luckily, most insects are not harmful to us or our plants but some of your garden visitors will be pests. Why do some insects become pests? There are several reasons!

Too much of a good thing
Populations of native Australian insects can explode when food availability is increased dramatically. A concentration of many thousands of food plants of the same type (i.e. a crop) is irresistible to plant-eating insects. Those native insects are then called “pests” as they affect crop yields, the profits of growers and the cost of produce to consumers. In Australia, such native insects turned pests include cutworms, armyworms, native budworm, lightbrown apple moth and grapevine moth. With a common name like grapevine moth you can be forgiven if you thought the last pest was not native. Don’t start me on stupid common names!

Grapevine moth larva

Grapevine moth larva

Monoculture modifies the environment to such an extent that it not only favours some insect species (pests) but inhibits others (usually beneficial insects or ‘good bugs’). For example, the native plant species which were in an area before a crop was planted may have harboured predators or parasites of a pest species – removing such plants may cause beneficial populations to collapse. In our gardens we can avoid this by growing a diverse range of plants, and not having too many of the same plant growing next to each other.

New kids on the block
When an insect is introduced into an area with few natural predators and parasites and an unlimited food supply, it can become a severe pest. Common introduced insect pests include diamondback moth, many aphid species, cabbage white butterfly, European wasp, African black beetle and potato moth. Common predators such as lacewings or ladybirds will often tackle a new insect but, if they are not in sufficient numbers and there are no pest specific parasites or pathogens in support, they may lose the battle.

Cabbage white butterfly

Cabbage white butterfly

The solution to this problem in commercial agriculture and horticulture has been to introduce parasites specific to a particular pest. A good example of that is the cabbage white butterfly for which a number of parasitic wasps was introduced back in the 1940s. If cabbage white butterfly arrives in your garden, chances are these wasps will follow them. When I was writing my book Garden Pests, Diseases & Good Bugs I left my brassica bed open to attack, rather than netting it off as I usually do, so I could photograph these parasites. Sure enough the cabbage white butterflies arrived as did their parasites and I got my shots. The interesting thing I noted was that towards the end of the season about 95% of the cabbage white butterfly larvae and pupae were parasitised. How’s that for biological control!

Killing beneficials
Pest populations can reach outbreak proportions if beneficial organisms are suppressed or eradicated. The best way to disrupt the activities of beneficial organisms is by killing them with pesticides. Broad spectrum pesticides kill beneficials just as efficiently as they kill pests. We really need to be careful with all chemical use if we are attempting to establish a population of ‘good bugs’ in our gardens. Some beneficial insects are even susceptible to ‘soft’ sprays such as horticultural oil and soap sprays – it pays to check before using such sprays and always carefully target the pest.

Parasitic wasps like this tiny cabbage aphid parasite are particularly susceptible to pesticides

Parasitic wasps like this tiny cabbage aphid parasite are particularly susceptible to pesticides

We have already seen how a change from native vegetation to monoculture can affect beneficial insect populations, but sometimes subtle changes can be just as damaging. Removal of border trees or hedges may remove resting and overwintering sites for the predators and parasites that help control pests in the crops they border, thus shifting the balance in favour of the pests. Parasitic wasps in particular need a nectar feed from flowers and, if no flowers are available, wasps may die out or leave the area. In a garden it is a no-brainer to have a range of flowers available to beneficial insects throughout the year.

Resistance to pesticides
This paragraph is for those who use synthetic pesticides in the garden. The process of natural selection among insects is continuing out there in the garden among your plants. Pesticide applications usually don’t kill 100% of the target pest, which leaves a small number of survivors who reproduce and pass on this ability to survive pesticides to some of the next generation. Resistance can develop and become entrenched very rapidly in pests that produce a new generation every few weeks. Not surprisingly pests with short life cycles and high numbers of generations per year top the list of pests resistant to the most chemicals. The two-spotted mite (below) which has a life cycle of 1 – 4 weeks and 12 – 20 generations per year (depending on temperatures) is resistant to over 70 of the chemicals registered against it. The green peach aphid is resistant to a similar number of chemicals.

Two-spotted mites aka red spider mite

Two-spotted mites aka red spider mite

Let’s tolerate a bit of damage
When I was a child finding a green caterpillar while shelling peas from the greengrocer was quite common (and not particularly alarming). The situation is very different these days. Produce buyers for the major supermarket chains appear to have a zero tolerance for any living thing in produce (including beneficials like lacewings and ladybirds) and damage to produce (even minor blemishes).

The same level of tolerance (or lack of it) to insects also applies in some flower markets. It is understandable that many commercial growers are reluctant to give up the spray regimes they have traditionally used to cater for this zero tolerance. The problem with that approach is that such a regime increases the likelihood of pesticide resistance. Frequent chemical use is also costly, not to mention the associated environmental and health concerns.

Minor caterpillar damage on gum leaf

Minor caterpillar damage on gum leaf

In a garden a rigid spray regime would be a ridiculous approach to adopt. We are not commercially bound to our produce like a commercial grower is, so why not tolerate a bit of a nibble or blemish here and there.

The future
The impact of climate change on pest insect populations is still largely unknown. There is no doubt though that climate change will favour some insects and inhibit others. If climate change favours pest species and inhibit beneficials, we are in dire trouble. The extent to which a species can survive climate change may ultimately depend on their ability to evolve and cope with drier and hotter conditions. One thing is for sure, pests such as aphids and bush flies will arrive earlier each year!

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Denis Crawford

About Denis Crawford

Denis Crawford has studied, photographed and written about insects for more than 35 years. His background includes a decade in entomological research, and many years collaborating with an integrated pest management consultancy. Denis is author of Garden Pests Diseases & Good Bugs: the ultimate illustrated guide for Australian gardeners and co-author of Backyard Insects (soon to be released in an updated edition. See more posts on garden insects on his blog One Minute Bugs.

One thought on “Why do we have plant pests?

  1. helen on said:

    Wise words, Denis. As I responded in one of your earlier blogs, it’s amazing what emerges from leafy greens when I wash them in my kitchen sink! My garden has occasional pests, and whiteflies were bad last year. I do very occasionally spray individual plants with targeted, low toxicity pesticides such as natrasoap, and dust tomatoes religiously with sulphur. A dwarf pear has blister mite but the mites live inside the blisters most of the time, so no point sprayiung with any contact insectide. Hopefully they won’t spread to my espaliered pears; I’m keeping a close watch and will prune off any affected shoots on the latter. And rather than spraying with anything potent or systemic, I’ve decided to tolerate the infestation on the dwarf pear and spray in winter with lime sulfur, which is apparently the best control method. This targeted approach is definitely working in the garden because there are literally clouds of hoverflies – so many that you can hear them humming! – and ladybirds everywhere too (and frogs!). It’s really exciting to see the steady increase of these beneficial insects in the garden.

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