Plant Pests biology and control

1. RHS Level 2
Certificate
Plant Health Week 18 – Plant Pests,
their biology and control.

2. Learning objectives
3. Know the problems posed by pests in horticulture and ways
in which these can be minimised.
3.1 Define ‘plant pest’.
3.2 Describe the damage caused by rabbits, cabbage white butterfly
larva, black bean aphid, two-spotted spider mite, glasshouse
whitefly and vine weevil
3.3 Describe in outline the life-cycles of cabbage white butterfly,
black bean aphid, glasshouse whitefly and vine weevil.
3.4 Describe TWO different methods of reducing the effects of EACH
of the plant pests named in 3.2; methods should be selected from
more than one of the control options (chemical, physical, cultural
or biological) available.
3.5 Describe how a knowledge of the life cycles of the cabbage white
butterfly, black bean aphid, glasshouse whitefly and vine weevil
contribute to the success of their control.

3. Quiz
 10 questions – 15 minutes
 Test conditions please

4. Quiz answers (1)
1. (a). 2. (i) Encourage natural predators. (ii) Avoid over
fertilizing (iii) Use controls that do not harm predators
(iv) tolerate some damage.
3. Possible answers include – ‘Resistafly F1’ Carrots –
carrot root fly and ‘Sarpo Mira’ Potato – late blight.
4. Chemical, biological, cultural and physical
5. Benefit – timing of control precise, Limitation –
resistance of some insects
6. Vine Weevil
7. One that produces several generations a year – e.g.
Cardamine hirsuta

5. Quiz answers (2)
8. (i) they reproduce vegetatively (ii) they have
extensive root systems.
9. Glyphosate – translocated or systemic.
Dicamba – selective
Diquat – contact
Metosulam – residual
10. (i) they compete with cultivated plants for
nutrients, water and light. (ii) they are unsightly (iii)
They can be hosts for pests, diseases and viruses

6. Plant pests
 Definition: any insect or
animal that causes
unacceptable levels of
damage to cultivated
plants, making them
unsuitable for the purpose
for which they are grown.

7. Pest Lifecycles and why
they matter
 Two basic lifecycles for insect pests.
 One is a complete metamorphosis. That is
Egg → larvae → pupa → adult
 The other is called incomplete metamorphosis.
That is Egg → Nymph (an immature adult) →
adult. There may be several nymph stages.
 Some pests are most effectively controlled at
one stage of their life cycle. For example Vine
Weevil are generally controlled as larvae, as
the adults are hard to find and the eggs and
pupae are not affected by controls.

8. Complete metamorphosis

9. Incomplete Metamorphosis

10. Rabbits
 Damage – nibbled plants
and damage to bark of
woody plants and trees
 Control – trapping (must
be done humanely);
physical barriers; repellent
sprays.

11. Cabbage White Butterfly
 Caterpillars do the damage – large holes in
leaves, Small Cabbage White caterpillar
also tunnels the hearts of cabbages.
 Full metamorphosis – adults, eggs and
pupae are hard to control, caterpillars are
vulnerable.
 Controls – physical barriers to prevent egg
laying (e.g. fine net); biological control -
Steinernema carpocapsae; chemical control
– pyrethrum or deltamethrin.

12. Black Bean Aphid
 Clusters of fat black insects at the growing tips and
in the leaf axils of plants.
 Life cycle – eggs laid on Euonymous europaeus
and related plants, young hatch and mature there in
several nymph stages, feeding on new growth.
Winged adults migrate in May/June to legumes.
Reproduce asexually all summer then produce
winged males which migrate, with the females, back
to the winter host.
 Damage – weaken the plant, excrete honeydew,
spread viruses.
 Controls – Fatty acid or pyrethrum sprays; autumn
sowing of Broad Beans (Vicia faba) and pinching
out tips in late May. Ladybirds and their larvae.

13. Two Spotted Mite
 Tiny eight legged mite, two darker spots on its
back. Overwintering females turn red.
 Life cycle: Egg → Nymph (3 nymph stages) →
adult. 7-14 days to complete.
 Damage: white speckles on leaves that gradually
taken on a bronzed appearance; webs or groups
of mites visible beneath in severe infestations.
 Controls: increase humidity; Fatty acid sprays on
food crops; Lambda-cyhalothrin on
ornamentals; Phytoseiulus persimilis is the
biological control under glass.

14. Glasshouse White Fly
 Tiny white sap sucking winged adults and
scale like nymphs.
 Lifecycle: Egg → Nymph → forms a legless
Scale → adult. Asexual reproduction.
 Damage: spreads virus, excretes honeydew.
 Controls: no effective chemical control other
than fatty acid spray. Encarsia formosa is
the biological control under glass.

15. Learning outcomes
3. Know the problems posed by pests in horticulture and ways in
which these can be minimised.
3.1 Define ‘plant pest’.
3.2 Describe the damage caused by rabbits, cabbage white butterfly
larva, black bean aphid, two-spotted spider mite, glasshouse
whitefly and vine weevil.
3.3 Describe in outline the life-cycles of cabbage white butterfly, black
bean aphid, glasshouse whitefly and vine weevil.
3.4 Describe TWO different methods of reducing the effects of EACH
of the plant pests named in 3.2; methods should be selected from
more than one of the control options (chemical, physical, cultural or
biological) available.
3.5 Describe how a knowledge of the life cycles of the cabbage white
butterfly, black bean aphid, glasshouse whitefly and vine weevil
contribute to the success of their control.