Dendrobium beetles

reproduced with permission from Cumberland Orchid Circle

The dendrobium beetle (StethopachysFormosana) is perhaps the most destructive of orchid pests for orchid growers, particularly those growing Australian native Dendrobiums. This beetle feeds on both the leaves and the flowers of orchids and what it leaves is disfigured and unattractive.

The Dendrobium beetle is a leaf beetle of the family Chrysomelidae, which is one of the largest beetle families with approximately 3000 Australian species, at least 250 of which occur in the Sydney area. (*JB: it is now permanent in the Sydney area) Like most leaf beetles, it is colourful and both the adults and the larvae eat plant material.

The beetle is a native of Australia. It is found in the northern part of the Northern Territory, throughout Queensland, and northern New South Wales. They can also survive further south into the Sydney region in the protection of shadehouses. Their preferred food is the native Dendrobium (all species and hybrids) but will also feed on many other species of orchid (JB: Cymbid flowers, Epidendrumetc).

Life Cycle: The beetle is quite distinctive and easily identified. It is about 12mm ( ½”) long, similar in size to a lady beetle. They are orange in colour with 4 black spots on their wing covers that may sometimes appear to be 2 black stripes. They also have large black antenna. The adult beetle feeds on leaves, flowers, buds, and seed capsules of the orchid. They are especially efficient at destroying all the forming flower buds if not controlled.

The adult beetle lays its eggs on new growths, buds, and flowers. The eggs are very tiny, elongated and greenish cream in colour. After about 10 days the beetle larvae (left) emerges and tunnels into the new shoots, causing them to turn soggy and die. While within the cane, the larvae will grow to a length of about 10-12 mm, by which time they will have eaten out the fleshy inside of the cane.

The larvae next pupates in a mass of white waxy material similar to toothpaste before the adult emerges. The pupae case (right) is usually located at the base of the canes among the surface roots and is 10 to 20 mm long. the material it is made of looks like some kind of polystyrene foam spaghetti. The life cycle of this beetle is only a couple of months. Generally this is during the warmer months (Dec to Feb) but you will occasionally find a beetle in winter if it is mild.

Control : One method of controlling this pest is to ensure that your orchid house is sealed, preventing the beetle from getting in, but this is much easier said than done. Although the beetle is quite easily controlled with most insect sprays, the larvae is very difficult to control. Most poisons that are effective on the larvae are also toxic to yourself, the orchids, and the environment; any others will not control the larvae well. It has been said of recent times that the systemic insecticide Confidor will work but I cannot confirm this.  (PS from Jim :Captan, which is only a contact spray, has also been said to kill what it hits, and the residual Captan smell supposedly also deters them for some weeks.)

Supplementary Observations by Jim Brydie :

The damage done by the adult beetle is the first thing you will see. Apart from chewed flowers and buds, these powerful little chompers generally strip sections of the surface layer of leaves in a very distinctive way. They don’t eat a hole right through the leaf. They eat the fleshy material off the surface leaving the woody inner layer intact, like a dead skin.

The adult beetles also have a very interesting defence mechanism that makes them easy to catch. If you put your hand (or a finger) near them they ALWAYS fold up their legs and let themselves drop. The trick is to cup one hand under them and then poke them. When they drop into your cupped hand you can drop them into a container of water to drown but I find it more satisfying to set them onto a hard surface and stomp the little sods. Oh, and please also note that adult beetles nearly always travel in pairs. If you find one, don’t stop until you find the second one.

If you catch them before they lay eggs, you don’t have to worry about the larvae but if you are too late, then beware that the larvae do also strip leaves. Perhaps this is before they go into the phase of burrowing into the stems. I once had a plant of Dendrobium delicatum being massacred on 20 or 30 leaves simultaneously. I thought it was adult beetles at first, as the larvae were so tiny I didn’t even see them until the second or third day. There must have been dozens of them and they made a frightful mess before I figured out what was going on.

If the larvae get inside the stems then those stems are more or less history as you won’t find the pest until they damage the stem beyond repair. The critical thing here is to make sure you don’t let the cycle progress through the pupae stage. If the stems are “mushed”, cut them off and put them in the bin. If you can find the critter inside then kill it. Keep a close eye on the plant and make sure you remove and destroy any pupae cases that appear, before they hatch out. The pupae cases are very distinctive and easy to find. They look like small pieces of that foam packing material we call “spaghetti” twisted together. They will be pretty well stuck to the base of the stem but they aren’t that hard to remove and squash them.

Editor’s note:  An old subject but it is a good article, so I thought it appropriate to publish it…Special thanks to Jim Brydie as this was published in Kuringgai Orchid Society Newsletter.