PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Scientific name
Acanthoscelides obtectus, Family Chrysomelidae,
Order Coleoptera. Note that although this insect is
called a weevil it is not a true weevil.
Host range
Beans and cowpeas in the field and in storage.
Description & damage
Weevils (adults) are small, stout, oval beetles
approximately 3 mm in length. They have white,
gray, brown or black patches on the upper surface.
The legs and antennae are reddish. Larvae. The
1 st stage larvae have legs, and move through the
pods and bore into seeds. Later stage larvae are
white and legless and grow to a length of 3 mm.
Seed. Damage is caused by the larvae feeding
and developing inside the seed both in the field
and after harvest in storage. After pupation of
larvae inside the seed, emerging adults leave round
exit holes. Infested seed is rendered unfit for
human consumption and seed germination may be
seriously affected. Infested seed that has not been
treated may be found to be riddled when needed
for planting.
Diagnostics.
Circular holes on seed.
In lightly infested seeds all stages are difficult to
find. A hand lens is needed to see adults feeding.
Can be difficult to identify one species of weevil
from another. Seek expert advice (page xiv).
Fig. 78. Bean weevil (Acanthoscelides obtectus).
Left: Adult about 3 mm long. Photo NSW Dept of Industry and
Investment. Right: Cavities produced in bean seed by
larvae covered by thin circular caps of skin. PhotoCIT,
Canberra (P.W.Unger).
Bean weevil
Not really a weevil
Table 16. Bean weevil – Some insecticides.
Pest cycle
There is a complete metamorphosis (egg,
larva, pupa, adult), with up to 6 generations each
year. Adult females may lay several hundred small
white eggs on bean pods or on split seed on the
plant. After hatching, larvae enter and feed in the
seeds, damage in the field is not usually noticeable.
Larvae complete their development inside the seed
after the seed is harvested. When fully grown, the
larva excavates a chamber near the surface of the
bean and pupates. A visible circular cap of skin
covers the chamber and is broken when the adult
beetle emerges leaving a circular hole. Beetles that
emerge in storage from field-infested seed, lay eggs
on other bean seeds in storage and on bean pods or
exposed seeds on plants in the field.
‘Overwintering’
All stages in stored seed.
Spread
By adult beetles flying, adults can invade bean
crops from where seed is stored.
By the movement of infested seed.
Conditions favoring
Warm, dry conditions for field infestations.
In storage they may breed throughout the year.
Management (IPM)
Are you a commercial grower or home gardener?
1. Obtain/pepare a plan that fits your situation.
2. Crop, region. Recognize variations.
3. Identification of pest must be confirmed. Consult a
diagnostic service if necessary (page xiv).
4. Monitor pest and/or damage and record results
(page 39).
5. Threshold. How much damage can you accept?
6. Action. Take appropriate action when any threshold
is reached improving sanitation methods, etc.
7. Evaluation. Review IPM program to see how well
it worked. Recommend improvements if required.
Control methods
Cultural methods. To minimize infestation
of seed in field, harvest as soon as seed is mature.
When beans are harvested for seed they should be
bagged as soon as dry and then treated if necessary.
Sanitation. Destroy residues of old infested
crops, seed residues and trash in boxes and sheds.
Biological control. No biological control
agents are available for purchase and none have
been released by government agencies. Little
appears to be known about natural controls.
Pest-tested planting material.
Check seed with a hand lens prior to planting.
Insecticides. See Table 16 below.
What to use?
DUSTS & SPECIAL PACKAGING
Dusts do not kill larvae or pupae inside the seed but they do
kill the adults after they emerge preventing further infestation.
FUMIGANTS
Group 24A (pages 60, 267)
When and how to use?
Use only for seed known to be clean or lightly infested.
After treatment, store in beetle-proof sacks or muslin bags
to keep seed free from infestation for a long time.
Insects and allied pests - Coleoptera (beetles, weevils) 113