PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Pest cycle
There is a gradual metamorphosis (eggs that
rarely hatch, live nymphs and adults) with many
generations each year. Survival of the population
during winter depends mostly on young aphids in
cracks and crevices of old wood. Most aphids are
wingless females that reproduce asexually (without
being fertilized) and give birth to between
2-20 live female nymphs a day. These take from
8-20 days to mature. Once aphids settle at a
feeding site they remain there until autumn. A few
sexual forms appear on apple but they do not feed
and their eggs rarely hatch. Some migration from
trees to susceptible rootstocks takes place in early
winter with a return to aboveground parts in
spring. On roots and aerial parts reproduction
continues through winter though at a reduced rate.
‘Overwintering’
Mainly as young aphids in cracks and crevices
on the above ground parts of apple trees and on
exposed roots or those less than 10mm below the
soil surface.
As young aphids on the roots of susceptible
rootstock but these aphids do not appear to be
important for ‘overwintering’.
Woolly aphid infestations survive better in the
field during winter on Granny Smith and
Jonathan than on Delicious or Rome Beauty.
Spread
Occasionally young winged adults, produced
during summer, establish colonies on
neighboring trees.
Movement of infested propagating stock, nursery
stock and plant material.
By wind, birds, insects or people.
Conditions favoring
Spring and autumn, eg 6-18 o C temperature.
Low humidity and temperatures above 27 o C are
unfavourable to it, but low temperatures have
little effect apart from slowing down their rate of
development.
Cool and moist conditions, eg shaded
situations, the interiors of dense, strongly
growing trees or trees shaded by windbreaks.
Lack of predators, parasites and diseases which
attack woolly aphids (Asante 1999).
As aphids become more resistant to pesticides
they become less sensitive to their environment.
Management (IPM)
Are you a commercial grower or home gardener?
1.Access a plan that fits your situation. State
Orchard Plant Protection Guides available.
2.Crop, region. Recognize variations.
3.Identification of pest must be confirmed. Consult
a diagnostic service if necessary (page xiv) to avoid
confusing waxiness, sooty mould and honeydew with
other pests.
4.Monitor pest and/or damage and record results
weekly (page 39). Seek advice for your situation.
Early detection before the ‘waxy wool’ is
produced in quantity is important as it protects the
aphids from insecticide sprays.
It is easier to count colonies to estimate population
numbers than to tally individual aphids.
Brough et al (1994) suggested inspecting 5 laterals
from each of 20 trees per hectare during spring and
autumn.
5.Threshold. How much damage can you accept?
Thresholds have been determined for some commercial
growers, eg if one (1) or more laterals are found to be
infested.
6.Action. Take appropriate action when any threshold
is reached, preferably that which does not harm
natural parasites and predators. A resurgence of aphids
may occur prior to harvest.
7.Evaluation. Review IPM program to see how
well it worked. Recommend improvements if
required, eg use of resistant rootstock for new
plantings so that woolly aphids will no longer multiply
on and damage roots.
Control methods
Control is difficult because susceptible rootstock
have a more-or-less permanent infestation on the
roots, which may re-infest aerial parts.
Biological control. The introduced wasp
parasitoid (Aphelinus mali) and native species of
ladybird beetles will attack woolly aphid but their
populations may not increase rapidly enough to
prevent woolly aphid populations reaching
damaging levels.
Introduced Aphelinus. Woolly aphid has been
under biological control since 1923 when the
parasitic wasp (Aphelinus mali) was released.
The wasp lays eggs in the body of the aphid and the
larva feeds on the body of the aphid killing it.
Parasitized aphids lose their woolly covering and
become black. A small exit hole which can be
seen with the naked eye, is made in the back of the
dead aphid by the wasp when it emerges. The wasp
'overwinters' as a pupa in the body of a dead
parasitized aphid and the adult wasps emerge in
September at the same time as the colonies of
woolly aphid become active. It is possible that
dormant sprays do not affect the protected stage of
the parasite but many of the more modern
insecticides used during the growing season are
harmful, the wasp is therefore scarce in
commercial orchards but gives useful control in
non-bearing orchards where fewer pesticides are
used. A. mali can complete 8-9 overlapping
generations each year. Conserving Aphelinus mali:
– Occasional infestations on hawthorn and other
hosts are valuable for maintaining Aphelinus
populations. Hawthorn and other hosts near apples
should not be sprayed.
– Twigs with parasitized aphids may be collected
before winter and stored in a shed away from birds
and placed in infested trees in spring.
156 Insects and allied pests - Hemiptera (bugs, aphids, etc)