PLANT PROTECTION 4

PLANT PROTECTION 4 – How to Diagnose Plant Problems
PESTS & DISEASES
The following is a brief summary of the main types of pests and diseases and some common examples of their
signs (physical evidence of the pest) and symptoms (visible reaction of the affected plant).
INSECTS &
ALLIED PESTS
Cabbage white butterfly
Some insect problems
are easy to identify.
You can actually see
the insect doing the
damage. Sometimes,
though the insects are
not present, eg they
have eaten their fill
and flown away, they
are overwintering
somewhere or they are
too small to be seen
without a hand lens.
Some insects like to
bore into stems or
roots where they can
cause damage without
being seen.
Chewing
mouth parts
Piercing & sucking
mouth parts
Parasitic pests & diseases
Phylum Arthropoda. Adult features include:
1. Body is divided into segments.
2. Hard outer covering on body and limbs,
with flexible joints for movement.
3. Paired limbs.
4. Bilateral symmetry (each side of the
body is a mirror image of the other).
Insects
1. Three body segments.
2. Three pairs of legs on thorax.
3. Antennae present (1 pair).
4. Wings either present or absent.
Mites
1. Two body sections.
2. Four pairs of legs.
3. No antennae.
4. No compound eyes, simple eyes present.
6 Background briefing – Causes of plant problems
Insect Mites Spider Springtails
Millipede Centipede Slater
Phylum Arthropoda - Insects & allied organisms,
remember, many are beneficial
Signs (physical evidence of the pest)
Insects themselves, nymph and cast skins.
Spittle on shoots that shelters nymphs of the spittle bug.
Wax secreted by mealybugs (white cottony material).
Honeydew produced by some sap sucking insects and the sooty mould growing on it.
Frass, insect droppings.
Webbing produced by spider mites, silk produced by caterpillars.
Wood moth caterpillar – wattle
(H. J. Elliott)
Vine hawk moth caterpillars –
impatiens
Whiteflies Lace bug 3 tiny thrips
Aphid Leafhopper Mealybug
Symptoms (visible reaction of the affected plant)
In the absence of insects or mites, symptoms may be observed. The type of feeding
damage depends on the type of mouth parts.
Chewing mouth parts may result in:
– Chewed leaves, flowers, buds, seeds (caterpillars, beetles).
– Skeletonization of leaves (pear and cherry slug).
– Tunnels in leaves (leafminers), fruit (fruit borers), stems and branches (borers).
– Girdled or dead stems (cutworms, twig girdlers, stem borers).
– Dieback and general decline of plants due to root damage (soil-dwelling insects) or repeated
defoliation (caterpillars, leaf beetles, sawfly larvae).
Piercing and sucking mouth parts may result in:
– Leaf stippling (leafhoppers, mites), distortion (aphids) or rolling, galls, defoliation.
– Wilting of shoots (various bugs), stunted new growth.
– Dieback of shoots, twigs or branches (scales).
– Discolouration of leaves, blossoms and fruit (scale).
Rasping and sucking mouth parts may result in:
– Leaf silvering (thrips), rolling, curling, galls.
Virus diseases transmitted by sucking insects.
Chewing damage by wood moth
larvae – tunnels in wattle roots
Chewing damage by caterpillars
– geranium
Stippling caused by piercing and
sucking spider mites – French bean