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Beetle grubs

Published on December 4th 2020
When we work in our gardens, we often find these big, creamy-white grubs, often referred to as worms, in the soil or in compost. In Afrikaans, we call them ‘miswurms’. They are actually not worms, but beetle larvae of the Scarabaeidae family. This beetle family contains all the scarab and chafer beetles.
In most gardens, these grubs will either be of the Fruit Chafer beetle), Pachnoda sinuata, or the Rhinoceros beetle), Oryctes boas, or the Black Maize beetle), Heteronychus arator. It is not always possible to distinguish between the species in their larval stage, because many species look very similar.
Larva of the Garden fruit chafer, _Pachnoda sinuata._
An adult beetle, frequently seen on roses and fruits.
The Rhinoceros beetle, _Oryctes boas_. C-shaped grubs - also often found in compost/garden beds.

Lifecycle of a grub

Recently, fellow gardener @jbrink shared an awesome post on grubs and their lifecycle in an average South African garden.
"The adult beetle will lay its eggs in soil when it is time to reproduce. Eggs would then hatch into Grubs, and grow to a stage where the next lifecycle kicks in, and it's time for the grub to turn into a beetle. During this lifecycle stage, the grub would cocoon itself with the surrounding soil, into a semi-hard shelled ball, in which the transformation takes place. One can identify this stage in shallow pots, by looking at the soil top-layer. It would resemble small and beady clumps of soil."
A bird sitting on top of a dirt field
One can see the clear lifecycle from Beetle to Grub of a Fruit Chafer Beetle. Photo by @jbrink
A insect on the ground
White-Spotted Fruit Chafer | Mausoleopsis amabilis. Photo by @jbrink.

Grubs - good or bad?

The most common question gardeners would like an answer to is whether these grubs are good for your garden or compost, like earthworms. Grubs do feed on the same food sources that earthworms do i.e. rotting plant material, and therefore you could expect that the excretions of both will be good for compost. However, grubs also feed on plant roots, especially that of seedlings or young plants and are thus not good for your garden. In addition, people sometimes assign signs of damage with plants to other organisms, while these grubs were actually the culprits. Although these grubs are not considered as a significant garden pest, in the agricultural world, some grub species pose major threats to corn in South Africa, including Anomala ustulata and Schizonycha spp.
A close up of an animal

How do I control them?

The Fruit Chafer beetles can be controlled by hanging baited traps, containing sugar water with a sliced banana fruit. It will attract the beetles and they will drown in the sugar water. Karba can be sprayed on flowers, which is a contact insecticide. If you have chickens, they can help to take care of the grubs or garden birds like the hadeda-ibis also feed on these grubs. So the best thing to do is remove them from your soil and then just put them somewhere visible away from the garden for birds to find.
Check out the Insect profile for Scarab Beetles and white grubs:
A macro shot of a Scarab beetle from the family Scarabaeidae

Scarab Beetles


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