Rhinoceros Beetle
- GardenMaestro
- Jan 29, 2023
- 3 min read
Updated: Feb 9, 2023
Common name: Rhinoceros Beetle
Order: Coleoptera
Family: Scarabaeidae
Subfamily: Dynastinae
Physical description
There are a number of species of Rhinoceros Beetle, but they all have similar general characteristics.
Large, shiny dark bodied beetles with a distinctive “Rhinoceros” horn on the front or top of the head. Adults can measure between 2 and 4cm in length depending on age. They have extremely strong biting mouthparts and cause serious damage both in the adult and larval stages. Though ungainly, these beetles are able to fly. The lifecycle of the beetle is between 5 and 36 months.
Description of damage
Larval stage damage is less apparent as it takes place in the root system or stem of the afflicted plant. Damage only becomes noticeable once the adult bores its way out of the stem (damaging the plant in the process) or soil. This forms a round hole up to 1cm in diameter through the soil, stem, or fronds of the infected plant. These beetle have exceptionally strong mouthparts and are able to eat much tougher plant materials that other insects would avoid.
These beetles normally only eat dead or decaying wood, as such have a tendency to attack plants that are damaged, dead, or already stressed, infected by another pathogen, or exhibiting signs of decay. For example, the root system of a plant that is being over-watered may start to decay, attracting the beetle adult to lay eggs in the proximity.
They are a particular problem on palm trees as they can damage the single, apical growth point of the palm – normally killing the bud in the process. Normally the exit hole extends through the soft-growing tip of the palm, damaging a number of fronds in the process.
The beetles are solitary/ singular, although on/ in heavily infected plants a number may be found.
The larvae (grubs) are often found in homemade compost or in the soil of plants that are being re-potted. These are large (up to 5cm long) and have a well-defined head.
Secondary infections (by other beetles and fungi) may cause serious damage to plants as well.
Plants affected
Almost all infections are related to outdoor plants, in particular, palm trees:
Washingtonia robusta - Mexican fan palm
Washingtonia filifera - Desert fan palm
Pritchardia pacifica - Fiji fan palm
Roystonea regia - Royal palm
Chamaerops humilis - Mediterranean fan palm
Phoenix roebelenii - Miniature date palm
They are also known to attack damaged Date Palms (Phoenix dactylifera), although the more common beetle pest in Dates is the Red Palm Weevil (Rhynchophorus ferruginous).
General comments
Unfortunately, this is one of those pests that you normally only observe the damage once it has been done and the perpetrator is long gone. It is important to check all garden palms regularly for signs of stress (wilting or curling of the apical shoots), exit holes in the trunk or near the base of the palm, etc.
Control methods - Natural
Adults are rarely seen during the day as they are predated on by birds and by bats in the early evening).
Control methods - Cultural
Beetles are nocturnal and are attracted to gardens and plantations by strong light sources at night; when the lights are turned off, they “go to ground” and look for a suitable source of food or a place to lay their eggs.
Light traps are a cheap and relatively effective method in reducing the adult population - although by this point they would have already caused the damage; nevertheless, this is a good way to prevent the problem from spreading.
Control methods - Chemical
If you have any of the "target" species growing in your garden that seems to be exhibiting signs of stress, an early preventative application of contact and systemic insecticide can be used from January onwards and repeated every month till May. This will either kill the beetle larvae on contact or poison the sap of the palm, killing the larvae while it feeds on the palm.
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