Ian Stauton

Beetles which eat your avocado, macadamia and mango flowers are eating your cash.

Same goes for tropical fruits; rambutan, lychee, durian, mangosteen, jackfruit – any and all of them.

But you probably already knew that.

Beetles have the right equipment; wings to fly into your tree, chewing mandibles and the inclination to inflict maximum damage. An eaten flower is a lost-to-you fruit to harvest, pack and sell.

Beetle adults and their larvae both have chewing mouthparts to eat crop-plant tissue… flowers, buds, roots and leaves.

Some beetles even pretend they are on your side by wearing ladybird orange and black markings.

But they’re not, they are leaf-eaters instead of insect eaters like ‘proper’ ladybirds.

Most pest beetle larvae are root eaters, and you may know them as ‘curl grubs’, found in the soil. Beetle grubs have three pairs of legs at the front. They breathe through spiracles which look like oval portholes along their sides.

Weevils are part of the beetle group; but their larvae are legless (even without booze) and they can be found in the rootzone and also inside plant tissue.

Monolepta, also known as the red shouldered leaf beetle, also love flowers. In just a couple of hours, beetles by the thousands can emerge from their pupal cases in nearby pasture, fly the few hundred metres to your trees and begin devouring the flowers you saw that afternoon which potentially represented thousands of dollars of fruit sales. Sound a bit dramatic? Maybe, but not to those growers who have experienced it. The ‘trigger’ for their swarm is wait a week or three after heavy rain. It gives trees time to put on new growth.

Eggs are laid in pasture, and maybe the grass between the rows, where there are a lot of fibrous roots just below ground level. As the grubs finish eating they go into the next, pupal stage, and once the change is complete, the new beetle can stay there in pause mode awaiting spring rains that produce new flowers on your tree crop. Smart, eh?

Cockchafers and Christmas beetles are scarabs, which means the adult beetles have a hard shell on the thorax butting up to hard shell forewings and with front legs designed to dig through soil.

Grubs of the scarab group are curled like a ‘C’.

Eggs are laid in the soil in summer so the hatching larvae can eat those fibrous roots.

The life cycle is usually two years but could be 3-4 years. Like the Monoleptas above, they can stay in the pupal stage until good rain promotes flowers and new leaves.

The Rhinoceros Beetle is also a scarab – but big.

The male has a couple of big projections on its head and thorax which it can move together like tweezers (I haven’t seen it holding anything). Males are 60-70mm long, females 30-40mm long. They chew fruit and bunches of fruit, and the dripping juices can spoil fruit below.

They are partial to lychees, other tropical fruit and even damage the bark of trees. Being more prevalent in warmer, sub- and tropical climates they also have a faster lifecycle of a bit less than a year. Eggs and immature stages are spent in rotting vegetation. (Be careful what you mulch for).

We still haven’t covered all the swarming leaf beetles; there is another group: Chrysomelids (I easily recognise them by their relatively big feet).

These are leaf-eaters about 3-5mm long, usually shiny black or brown and the last, ‘foot’ segment on each leg is larger and flat… reminds me of a small snowshoe.

Don’t think they are cute; if you are growing fruit, they know where you live.

Again, their larvae are white grubs feeding on fibrous roots… only smaller. The beetles hide in the shady underside of leaves by day and feed all over the tree by night.

Leaf-eating ladybirds are serious pests of vegetables, less of a problem in tree crops.

Their yellow eggs are laid on either side of leaves in groups and the hatching larva, with its branched spikes, wanders around etching the surface leaf tissue between the veins… a bit like kids leaving their crusts! Good ladybirds eat aphids, whiteflies, thrips, young scale insects and the eggs of moths/butterflies that would have otherwise become leaf-eating caterpillars.

Good ladybirds have fewer spots, some have stripes; the ‘outlaws’ have 24-28 spots.

If you see ladybirds on your crop, look around for the larva; the ‘goodies’ are usually red/black and yes, they also have spikes. If there are no etch-damaged leaves, Biological control could be in full swing.

Control

If the nearby pasture or sugar cane is yours, you might consider some residual soil insecticides to eliminate or reduce larval stages. Systemic insecticides which travel around the sap stream will kill pests in the early part of the season while the fruit is well short of the crucial withholding period.

Residual insecticides also have a withholding period to be aware of and you have to recognise that, as new shoots are a constant event, the residue stays where it was sprayed but the arriving pests go to the new growth. Knockdown pesticides kill all the insects you contact at the time… and have just a one-day withholding period.

What do you do when flower-eating beetles such as the monoleptas arrive and bees are also wanting to harvest nectar from the same flowers?

You spray at dusk. The bees are back in their hive; the leaf/flower-eating beetles die within minutes and next morning, if you’ve used a natural pyrethrum spray which is repellent (like Aerogard) to bees, they won’t go near the flowers until about morning tea time next day when the UV light has degraded all the toxicity and repellency.

Then it’s business as usual for the bees… with more flowers, because the beetles haven’t eaten them.

Footnote on floods. If beetle grubs were eating pastures and crops which have been underwater for say 2-3 days or more, the grubs (not having snorkels) will drown. I suggest you could spend a moment with a spade and dig up some pasture to see if the curl grubs are dead. That’ll be a bit of good news!

Ion Staunton is an entomologist at Pestech Australia P/L makers of PyBo Natural Pyrethrum Insecticidal Concentrate. Questions? 180012345 7 or pestech.com.au