Pests of chrysanthemums

Note Number: AG0516
Published: June 2000
David Williams and Kathy Pullman, Knoxfield

Chrysanthemums are attacked by a variety of chewing, sucking and rasping (scraping) pests. To make problems worse, chrysanthemums are sensitive to some pesticides, such as those containing dimethoate, and some cultivars may be sensitive to pesticides containing dicofol.

This Agriculture Note provides you with a general description and the biology of major chrysanthemum pests.

Chewing insects

Budworms (Helicoverpa spp.)

Budworms are the larvae of nocturnal (active at night) moths. Species which attack chrysanthemums in Victoria are the same ones that attack tomatoes and sweetcorn. Budworm larvae feed mainly on very young terminal shoots, young foliage, flower buds and heads. Evidence of feeding can be seen by characteristic round holes in buds and flower heads.

Fully-grown caterpillars are about 40 mm long and may be various shades of yellow, green, pink or brown, with dark flecking and dark markings or stripes along their bodies. Caterpillars feed for two to three weeks in warm weather. When they have eaten enough, they leave the plant and burrow about 100 mm into the soil where they pupate.

Adult moths have a wingspan of about 40 mm and have grey-brown to reddish-brown forewings with darker markings. The hindwings are pale at the base and dark at the tips. Moths are inactive during the day and fly at night to lay eggs and feed on nectar. Each female can lay up to 1 000 eggs which are deposited singly on new leaves.

Cineraria leafminer (Chromatomyia syngenesiae)

This leafminer attacks a range of plants, including ornamentals, lettuces and weeds.

Severe infestation causes leaves to wilt and die. Larvae tunnel into the leaves disfiguring them. Tunnels begin as a grey or silverish-wandering line that gradually widens as the larvae grow bigger. The adult leafminer is a small black fly about 2 mm long. Females lay eggs in the undersides of leaves.

Chrysanthemum gall midge (Rhopalomyia chrysanthemi)

Severe infestations of gall midge can produce twisted stems and deformed leaves and flowers.

Larvae hatch from eggs laid on young growth and burrow into leaf tissue. After about one week, small patches begin to appear and cone-shaped galls (about 2 mm long) develop. The adult midge is a reddish-brown fly about 2.5 mm long. The chrysanthemum gall midge spreads from area to area via infested plants or cuttings.

European earwigs (Forficula auricularia)

Plants attacked include ornamentals, lettuces and vegetables.

Earwigs chew on flowers, buds, fruit and a wide range of living and dead plant and animal material. Caterpillar and earwig damage may be confused with one another, but they are easily distinguished because caterpillars leave trails of distinct pellets made of faeces.

Earwigs may be found in leaf litter, other debris, under loose bark and in piles of rock and timber. They like to crawl into tight places so that their bodies touch the surroundings.

Adult European earwigs are reddish-brown and about 20 mm long. Young earwigs are generally paler in colour than adults. Females lay whitish-oval eggs in batches of 20-50 at the bottom of a burrow in the top 50 mm of soil. They stay with the eggs until they hatch and for about two weeks afterwards.

Sucking insects

Aphids

The most common aphids infesting chrysanthemums in Victoria are the green peach aphid, Myzus persicae, and the chrysanthemum aphid, Macrosiphoniella sanborni. The green peach aphid is a green to greenish-yellow colour and the chrysanthemum aphid is a shiny mahogany colour.

Aphids feed on roses by puncturing and sucking plant fluids from the veins. These insect pests cause damage by secreting large amounts of honeydew which collects on leaves and buds. Honeydew encourages the growth of sooty mould which is unsightly and inhibits photosynthesis by reducing the amount of light reaching the leaves.
Only pregnant females survive over winter. In spring, females reproduce without mating and produce nymphs (immature stages) which form social broods and feed near their mothers. Winged females develop and disperse and aphid populations build up rapidly with spring growth.

Broad mite (Polyphagotarsonemus latus)

These mites attack a range of ornamentals and vegetables, and lemons and mandarins. Mites feed on the lower surfaces of young-soft leaves causing them to turn bronze and the edges to curl inwards and remain narrow. The leaves may also be brittle, puckered and curled. Plant growth is stunted and damaged buds often fall. Flowers may be deformed and discoloured. Broad mite cannot be seen without a microscope. Adults are similar in size to cyclamen mite, but are translucent and move actively over the plant. Eggs are covered in whitish lumps (tubercules).

Cyclamen mite (Phytonemus pallidus)

This mite attacks a wide range of ornamentals. Low infestation causes flecks with dark spots or discolouration on flowers. Severe infestation causes malformation of the flower buds which fail to open properly and wither and die. Infested leaves become distorted and brittle. As leaves unfold and get bigger, the mites move to new buds where it is more humid. Cyclamen mite is normally found inside leaf and flower buds. Sometimes older leaves are attacked and the mites feed on the lower surfaces causing the leaves to curl. Pockets may develop on the leaves and provide shelter for the mites. Adult mites are about 0.2 mm long. Females are amber or brown and oval shaped with two pairs of legs at the front of the body, which are widely separated from the two pairs of thin rod-like legs at the rear. Males have larger hind legs.

Oval-whitish eggs are laid in flower buds. Under ideal conditions, the life cycle of one generation can be as short as 10-14 days.

Two-spotted mite (Tetranychus urticae)

Chrysanthemums are damaged when nymphs and adults pierce the leaves and suck the contents of cells causing them to collapse and die. Heavily-infested leaves become lifeless and may become bronzed and shrivelled. Plants may lose their leaves and die.

Females lay up to six eggs a day, totalling 70 or more. Eggs are minute, globular and almost transparent pale-yellow-white. Males develop from unfertilised eggs and females develop from fertilised eggs. Egg hatch takes from three to 10 days depending on the surrounding temperature. Newly-hatched larvae have six legs, are pale yellow-white, minute and oval. When they moult, larvae become eight-legged nymphs which mature after another two moults. Young mites take four to 12 days to mature and live for about three weeks. In hot weather, the complete life cycle may be as short as two weeks and overlapping generations may occur.

Adult females are about 0.5 mm long and adult males are about 0.3 mm long. When feeding in summer, adult females are a yellowish-green with two pronounced dark spots, one on each side of the body. In males, these spots are less obvious and the body is smaller and tapered towards the rear. Both sexes also have two reddish eye-spots on the body.

Twospotted mite is a profuse web-spinner in its active stages. Under artificial long-day length or in warm situations, such as glasshouses, twospotted mite may continue to feed and reproduce through winter. Outdoors, when temperatures and day length decrease in autumn, males die and adult females stop feeding and change colour to orange-red. They usually migrate to sheltered sites where they assemble over winter. When temperatures increase in spring, females leave their shelters, begin to feed, and lose their red colour.

Rasping insects

Western Flower Thrips (Frankliniella occidentalis)

Western Flower Thrips (WFT) is transported in horticultural produce and is an insect pest of many ornamental, fruit, and vegetable crops.

WFT has been found in all Australian States, except the Northern Territory. This major insect pest is responsible for millions of dollars worth of crop damage world-wide due to its resistance to many insecticides and efficiency at spreading tomato spotted wilt virus.

WFT usually live and feed on flowers or new plant growth, such as buds and young leaves. Thrips have tubular mouthparts which cannot penetrate deep into plant tissue, so they usually feed on soft-recent growth or flowers by rasping or scraping surface cells and sucking the cell contents. Damage is not always obvious after feeding but becomes more obvious as the affected flowers, leaves or fruit grow and distort.

The WFT life cycle is mostly continuous and all stages of thrips can be found throughout the year. Female thrips live for 30 to 45 days and can produce around 150 to 300 eggs.

Diagnosis can be difficult because WFT looks similar to common onion and tomato thrips, however, experts from the Institute for Horticultural Development’s, diagnostic centre at Knoxfield can identify thrips specimens and give you advice.

Chemical control

Chemicals registered for use against major chrysanthemum pests may not be registered specifically for chrysanthemums but may be registered under "ornamentals."
Remember, you are responsible for ensuring that the product you are using is registered for the purpose you are using it for.

Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available at the Institute for Horticultural Development (IHD). For further information phone Crop Health Services on (03) 9210-9222 or fax (03) 9800 3521. For further information on registered chemicals, phone Chemical Information Service.