COMMON ORCHID PESTS: SYMPTOMS, DESCRIPTIONS, CONDITIONS and TREATMENT. Part 2. byJohnKrens
MEALY BUGS & SCALE INSECTS - Symptoms: Visible scale on the leaves or white powder left by the mealy bugs, as well as honeydew deposits and yellowing of affected areas, are the visible signs of these two pests. However, even after eradication and removal of scale, small pits will remain. Description: Both pests tend to hide under leaves and in leaf axils, while mealy bugs may also hide in the potting mix, thus making them hard to detect. Both have a protective coating that makes them difficult to kill. Prolonged infestation can affect plant vigour. Mealy Bugs and scale may also leave honey-like deposits that can initiate fungal infestations. Conditions: They like warm, mild conditions, although the mealy bugs don't like very hot weather. Spring and summer are the main periods of infestation in the shade-house but they may be a problem in the hothouse all year around. Treatment: Spraying with a systemic insecticide (one that is absorbed by plant tissues) will generally kill these pests but spraying needs to be repeated two or three times at intervals of 10-14 days to ensure that the following generations are also killed. Scale may also be treated with eco oil (or white oil mixed with malathion or a systemic insecticide). The oil suffocates these pests; when mixed with toxic substances, it also kills by poisoning. Oil may damage the foliage of some orchids - always water plants before spraying them. Watering with insecticidal soap while dislodging scale manually (eg. with sharpened bamboo stake) is also recommended.
MITES, TWO-SPOTTED MITE & FALSE SPIDER MITE - Symptoms: The damage from spider mite can often be seen as a discolouration (silvering of the underside of the leaves) for the two-spotted mite and as pitted depressions for the false spider mites. Severe infestations can lead to malformed leaves and flowers. Description: Two-spotted mites (0.5 mm long), often known as spider mite or red spider mite, will spin a fine web, which helps to detect them. False spider mites are only half the size of the two-spotted mite (left in picture). Both type of mite pierce the plant with their proboscis and suck plant sap, opening the way for fungal infections. You can normally not see the mites and have to deduce their existence from the symptoms. Conditions: Mites like hot dry weather, and only become a problem under those conditions. They hide in cracks during winter. Treatment: Mites can be treated with miticides such as Kelthane and Omite (carefully follow the instructions on the packs of these products). Avoid using the same product all the time to minimise the development of resistance. They may also be sprayed with a systemic insecticide or watered with insecticidal soap solution. Increasing the relative humidity and spraying affected plants with oil can fix light infestations and terminate over-wintering populations.
APHIDS (GREENFLY) - including WOOLLY APHIDS - Symptoms: Aphids are easily visible, especially on young growths, and are often visited by ants. They may cause deformities and yellowing of new growth. Description: Under favourable conditions aphids can multiply very rapidly, causing stress to the plant by their sheer numbers. The aphids prefer to attack new shoots and flower buds. Affected flower buds will be disfigured, if they open at all. Aphids also leave honeydew deposits, which tend to become mouldy as well as attracting ants that help the aphids travel to different areas of the plant. Woolly aphids have a waxy covering that gives them a similar protection against sprays as mealy bugs. Conditions: Aphids tend to appear when the plant is developing new growth and can multiply very rapidly. Treatment: Aphids are generally easy to treat, with anything from a mixture of soapy water and methylated spirits or insecticidal soap (and a little brush) to surface spray insecticides. Heavy infestations might require the use of a systemic insecticide.