No matter the crop, pests are the same

Feb. 25, 2023 | 5 Min read
The ‘Know your Pest’ article each issue was aimed at a pest and mentioned the crops usually under attack; this time we start with the crop(s).

You the reader and me, Ion Staunton*, the writer have a challenge this issue.

The editor decided to focus on ‘specialty fruit trees’; he lists Pongamia and Carobs (both legumes), persimmon, pomegranate, figs, coffee and possibly even dragon fruit (a cactus) under this grand-sounding title. So, let’s stick to basic principles which can be applied to insects attacking the above fruit trees… and the mainstream fruit crops.

(I know, you’re wondering what would attack cacti? Well, think caterpillars… remember the Cactoblastis moth was introduced from Argentina to control Prickly Pear? Bingo)!

The ‘Know your Pest’ article each issue was aimed at a pest and mentioned the crops usually under attack; this time we start with the crop(s).

Control is pretty similar for all tree crops; the more common citrus, nuts, pome and stone fruit trees just get more attention.

Let’s set out some basic principles:

1. All plants are threatened by an insect pest at some stage of the year.

- Insects which evolved to eat plants have the inclination to find one they like to eat

- These insects have a lifecycle which you should aim to break

- If the cost of fixing a problem is less than what it is likely to cost you if you don’t, that’s your judgment to make.

1. Pest threat

As the grower, it is your job to look at your crop more than occasionally to see if any pests are threatening its potential.

Apart from noticing live insects/mites, you should be looking for yellowed leaves, leaves with chewed edges, spotting on the stems and leaves where insects have poked a hole into the tissue to suck out sap (and maybe to introduce a virus). Also, galls can form in stems or leaves.

Typical life cycle of beetles, butterflies etc., from egg
through larva to pupa then adult. 

2. Breaking the life cycle

There are two main types of lifecycle:

a) Egg-nymphs-adult

The aphids, bugs, thrips, leafhoppers, mites and others lay eggs which hatch into a nymph and these nymphs shed their shell to get to be bigger nymphs until finally the last moult reveals an adult, ready to mate and lay more eggs. Generally, the nymphs don’t leave the plant where their egg was laid, so, a contact spray application will kill all the insects it hits… but that is not always the end of the infestation.

Eggs are sometimes well protected and will hatch soon after the initial spray. Another spray when the oldest nymphs are ready to become adults means all the eggs have hatched and there are no adults to lay more eggs, so the second application has broken the life cycle. BUT…

Thrips, for example, hatch into nymphs and the third nymphal stage, when it is fully fed, bails out to the soil. The fourth-generation nymph doesn’t eat but, down there in the soil it has plenty to do in transforming itself into an adult with wings and reproductive organs. This can take 3–6 days. Your first contact spray application will kill the adults and the first three nymphal stages (the correct word is ‘instars’).

Eggs are usually laid in slits in the plant tissue made by Mrs Thrip… so the contact spray has little effect on the eggs or the fourth instar nymph down in the soil. You need to spray again say 3–4 days after the first application to kill the newly hatched eggs and the newly emerged-from-the-soil adults before they can mate and begin the next generation.

As a tree crop grower (and Tree Crop reader) you surely know life isn’t meant to be all that easy because thrips can be carried longish distances on the breeze. A new invasion is always possible. Using systemic insecticides that run through the sapstream can be a better control method but read the label to observe the withholding period for your crop and whether the pest attacking your specialty tree crop gets a mention. The label is the law.

Contact killers i.e. pyrethrum with a 1-day withholding period have their place during the last weeks before harvest, or if you only need to apply to part of your crop.

Typical life cycle of bugs, cockroaches etc., from egg through nymphal stages to adult. 

b). egg-larva-pupa-adult

Beetles, moths/caterpillars have a different life cycle: Eggs hatch into larvae (or grubs) and when fully fed they usually leave the plant to go through the pupal stage to change into adults. Eggs are laid where the hatching larvae can get started on feeding immediately. Often this is in the soil (particularly some of the beetles whose larvae are curl grubs living in pasture eating grass roots).

Other eggs that produce caterpillars are laid on the host plant so junior can get straight into eating the leaves (or burrowing down into pome, stone or nut fruit). Most adult moths/butterflies do not feed on the plant (but see the rare photo of a fruit piercing moth being biologically controlled); adult beetles can fly in from surrounding pasture to eat the leaves and young flowers.

Breaking this type of life cycle is much more difficult! You could treat the soil in nearby pastures… but not if you don’t own it. You can spray the caterpillars on your trees to kill every last one… but moths and butterflies fly in over your boundary from distant parts and lay more eggs. Systemic insecticides have their place in killing anything eating your trees, providing you observe the withholding periods. Contact insecticides such as Py-Bo Natural Pyrethrum Insecticidal Concentrate is great at rescuing a threatening situation, but that takes you back to point No. 1… you need to look at you crop… often. But you do anyway, don’t you?

3. The cost

If it is shaping up to be a great crop, you won’t want to risk it. You may even have a contract or a buyer that you don’t want to disappoint… so the pressure is on. If there are beneficial predators eating pests and things aren’t serious, that’s OK. If the risk is increasing and it will cost you less to treat than you will lose if you don’t… your decision is simple.

*Ion Staunton is an entomologist at – manufacturers of Py-Bo Natural Pyrethrum Insecticidal Concentrate. Contact: 1800 12345 7.

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