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University
of Rhode Island GreenShare Factsheets
Plum
Curculio
Conotrachelus nenuphar (Herbst)
Coleoptera: Curculionidae
The plum curculio is widely distributed east of the Rocky
Mountains and is native to North America. The plum curculio
can be very destructive, attacking not only plums, but also
apples, peaches, pears, cherries, quince and other wild and
cultivated fruits. Injury results from the spring feeding of
adult beetles, from female ovipositioning in the fruit, from
the feeding of larvae within the fruit and finally from the
early fall feeding of adult beetles.
The adult plum curculio is a small, hard-bodied, brownish-black
snout beetle mottled with white and orange areas. It has four
prominent black humps on its top surface. It is about 6 mm
(1/4 inch) long and has a long snout, the end of which bears
chewing mouthparts. The fully-grown larva is about 9 mm (3/8
inch) long and is a yellowish-white, legless grub with a brown
head.
The plum curculio overwinters as an adult under yard
debris or in protected parts of an orchard. In spring, shortly
after peaches bloom or when apples are near the pink stage,
the beetles come out of hibernation and begin to fly to fruit
trees to feed. Egg-laying begins as soon as the young fruits
form and continues for three or four weeks. To lay eggs, the
female cuts a small, round cavity directly under the skin of
the fruit with her mouthparts. She then lays a single tiny
white egg in the cavity, just under the skin of the fruit.
Next she cuts a crescent-shaped slit nearly halfway around
the cavity, creating a dead flap in the surface of the fruit.
A single female lays about 60 to 150 eggs. Within seven days,
larvae emerge from the eggs and begin to eat their way through
the flesh of the fruit. In stone fruits, the larva works its
way toward the pit, around which it feeds extensively until
it is fully grown. On apples, few if any of the young larvae
survive if the apple continues to grow on the tree because
the egg or larva is crushed by the growing tissues of the fruit.
If the apple drops prematurely or is picked from the tree while
the larva is still alive, or if the variety is early-ripening,
the larva can complete its feeding and growth. The larva spends
about 10 to 16 days feeding.
When fully developed, the larva burrows out of the fruit,
making an oval exit hole, and enters the soil. In the soil,
the larva constructs a small cell in which it transforms into
a whitish pupa and then into an adult. The time between the
entrance of the larva into the soil and the appearance of the
new beetle above ground is about five weeks.
Summer brood adults emerge in July and
August. They do not lay eggs but instead feed on fruit, showing
a preference
for smooth-skinned fruits such as apples and plums. On apples,
the beetle makes a small hole in the skin of the fruit and
then devours all the flesh of the fruit it can reach with its
snout. This feeding takes place until the adults enter hibernation,
which occurs from September through early November.
Both the adult and larval stages
injure fruits. In spring, adults feed on buds, blossoms,
leaves and new fruits. Feeding
scars appear as shallow cavities on the fruit surface. The
major injury occurs from the laying of eggs by the curculios
(weevils) as described above. The early feeding and egg-laying
punctures can cause marked scarring and malformation of the
fruit. Early feeding on the surface of peaches often causes
severely deformed fruits known as "cat-faced" peaches. Larval
feeding in apples can cause distortion of the fruit. The mechanical
injury by adults in feeding and egg deposition can cause premature
fruit drop. When the summer brood of adults appears, feeding
cavities again can be found on the fruits.
Physical: Mechanical control--jarring the sluggish
beetles from trees in the morning and capturing them on sheets--is
practical on a small scale. Natural control of the curculio
results from winter mortality, attacks by birds and other predators,
and from parasites. Sanitation can also lessen damage. Pick
up fallen fruit two to three times a week and put it in a plastic
bag, tie the bag tightly and place it in the trash can. This
will help keep larvae in fallen fruit from developing in the
soil and, if done regularly, can minimize damage.
Chemical: On apples and
pears, apply an all-purpose fruit tree spray or the insecticide
phosmet (Imidan) when the
petals fall from the blossoms and again 7-10 days later for
the first generation beetles. Second generation beetles only
feed on the fruit skin and their damage can be tolerated. For
an overview of home apple tree management and sources of disease-resistant
fruit trees, see "Home Tree and Small Fruit Pest Management
Guide," or visit the URI Apple IPM website at http://www.uri.edu/research/IPM.
Adapted from the Ohio State
University Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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