COMMON ORCHID PESTS: SYMPTOMS, DESCRIPTIONS,
CONDITIONS and TREATMENT. Part 3. by John Krens
Pests covered in the third and final part of this series are Thrips, Woodlice, Millipedes & Earwigs, Ants, Codlin Moth, Dendrobium Beetle, Rats & Mice, and Possums.
THRIPS - Symptoms: A silvery lustre on the leaves (surface of cells sucked empty) with dark spots (excrements). Frequently seen on flowers; they often infest terrestrial orchids. Description: Thrips are greyish or yellow to brown and about 1-2 mm long, with fringed wings. The females lay their eggs in the leaf tissue, which hatch within a few days. The larvae are very mobile and start feeding immediately, extending their range below the potting mix surface, where they also pupate (life cycle takes from 12 days at 30°C to 20 days at 20°C). Thrips are believed to be instrumental in the transfer of viruses. Conditions: Thrips don't like cold weather and favour a hot dry atmosphere. Only a small number normally survive the winter but their numbers can build up in hot dry winter weather. They live on all types of plants but seem to prefer terrestrial plants, so keep weeds in the area under control. Treatment: Thrips can be treated with a range of insecticidal sprays, such as methidathion (Supracide®), dimethoate (Rogor®), malathion (also used for treating head-lice) and imidacloprid (Condifor®), at intervals of 2-3 months.
WOOD LICE (SLATERS), MILLIPEDES & EARWIGS - Symptoms: Wood lice, millipedes and earwigs hide in the dark cool places, for example, under pots standing on the ground (lift pots to check). They forage at night, so a nightly trip with flashlight could reveal their presence. Description: Wood lice (often referred to as slaters) are small multi-segmented crustaceans that, when feeling threatened, roll themselves into a little ball. Millipedes have long, narrow-jointed, black bodies. Woodlice and millipedes eat decomposing vegetable matter, including decomposing pine bark, thus causing orchid mixes to clog up. They also eat root tips when there are no better alternatives. Millipedes and earwigs eat small insects like thrips, but also eat root tips (especially those of epiphytic orchids) and flowers. They hide in places such as bracts, at the base of pseudobulbs, and in and under pots. Conditions: These pests like cool damp environments, so orchid pots are excellent places to hide as well as to feed. Treatment: Watering the plants with an insecticidal soap or use of insecticidal powder is recommended as treatment. Woodlice can also be controlled by snail pellets (place them under a pot to protect them from moisture), or by trapping them with potato slices (see cockroaches). Earwigs can also be trapped in balls of crumpled paper. Nightly flashlight expeditions and squashing works too! Keeping growing areas free of litter helps avoid these pests becoming a problem.
ANTS - Symptoms: Ants moving around on orchid plants, or trails of ants leaving or returning to orchid pots. Description: Ants don't eat orchids, but they can build nests in pots, seriously clogging up the orchid mix. They also move honeydew-creating sucking pests (such as aphids and mealy bugs) around, thus actively spreading pests. Conditions: Ants prefer dry cool places for their nests but spread their activities to any type of environment where they can find food. Treatment: Locate the nest and apply ant-bait, such as AntRid or AntSand, to exterminate the nest. If the nest is in a pot, you may need to re-pot that plant and clean the mix and treat it with a mix of pest oil and pyrefhrum to stop re-colonisation. Watering with insecticidal soap or use of contact poisons like Roach & Ant Killer (Yates) sprayed on pots, benches and walls, helps to control ant infestations.
CODLIN MOTH - Symptoms: Little brown caterpillar droppings or (if the pest is not discovered in time) collapse of flower spikes. Description: A small brown moth that lays its eggs on new growth and flower spikes. The small white/yellowish caterpillars that emerge from the eggs burrow into the new growths and flower spikes, causing them to rot and collapse. Until that time, the only outward signs are the small brown droppings of the caterpillars. Cymbidiums and native dendrobiums seem to be most affected. Conditions: This pest originates in fruit orchards and is a substantial problem in areas such as Melbourne's Eastern suburbs (former orchards) during the wanner months. Treatment: Regular spraying during warmer months (October till May) with carbaryl and a systemic insecticide (like Rogor® or Lebaycide®).
DENDROBIUM BEETLE (ORCHID BEETLE) - Symptoms: Little orange eggs, holes with waxy extrusion deposits, dying leaves, larvae and beetles scurrying around at night. Description: The orange beetle (about 10 mm long with black spots) feeds on orchids, with a preference for Australian dendrobiums. The larva is a slimy orange caterpillar that feeds on green tissue but also tunnels into new growths, causing them to die. Conditions: Dendrobium beetles are a serious problem from the northern Australia down to Sydney. Lower winter temperatures make them less of a problem in Melbourne. However, care needs to be taken not to import them into the hothouse with plants brought from infected areas. Treatment: Difficult to control. Squashing on sight and regular spraying with carbaryl when beetles are active. Eggs can also easily be removed by hand (when noticed).
WHITEFLY - Symptoms: Tiny white flies accumulating on young foliage, especially on underside of leaves, or on flowers. Description: White fly suck sap from plants, thus weakening them and possibly spreading diseases. Conditions: White fly can become abundant under mild weather conditions. Treatment: spray with malathion.
RATS & MICE - Symptoms: Sudden disappearance of floral segments of cymbidiums and/or greenhoods (mice), pots overturned and roots eaten or canes chewed through at base (rats). Description: Rats and mice tend to hide during the day and forage during the night. They are interested in everything edible. Under optimum conditions they can multiply explosively. Conditions: With cats being kept indoors during the night, rats and mice have become more mobile. Treatment: Use of poisoned bait (eg. Ratsak®, Ratsak Oneshot® - put inside an open container to reduce likelihood of unintended victims) will solve the problem, although this might take some time.
POSSUMS - Symptoms: Flowers being eaten. Description: Possums are nocturnal and dine mostly on vegetable matter. They like lemon (skin only) and oranges (inside only). Conditions: If no other preferred food sources available, possums may turn to orchids for food. Treatment: Have a preferred food source available. Lemon and orange trees with fruit all year around seem to prevent possums from becoming a problem.