|  COLORADO
POTATO BEETLE
Of
all the potato insects, the best known and most wide spread is the
Colorado potato beetle (Leptinotarsa decemlineata). The Colorado
potato beetle (CPB) was first observed in Nebraska and then identified
in Colorado. It is a well known pest in both commercial fields as
well as home gardens. Its host range encompasses all members of
the Solanaceous family, such as potato, tomato, pepper, egg plant,
and weeds such all nightshades and buffalo bur. It can be a pesky
pest defoliating whole potato fields in some parts of North America.
Description
CPB
undergo complete metamorphosis: adult, egg, larva, and pupa.
Adults
are hard-shelled with a round, convex shape. Their forewings are
yellow with a total of 10 black stripes running longitudinal. They
are about a half an inch long. Adults eat foliage until they pupate.
Eggs
are oval, yellow to bright orange. They are layed in clusters of
10 to 30 eggs on the underside of leaves.
Larvae
are slug-like with a soft shell. They are red to orange to tan depending
on age and they have two rows of black dots on each side. The body
which is humped enlarges with time and grows in four size stages.
Larvae eat foliage as they grow and this is the most destructive
stage.
Pupae
are small and are found in the soil .
Life
Cycle
Adults
overwinter four to 12 inch deep in the ground of harvested potato
fields and emerge in spring around May. Adults do not migrate but
will fly for several miles to find its Solanaceous hosts. Besides
potato, these include weeds such as nightshades and include garden
crops such as tomato and pepper. Adults then mate and lay eggs on
the host plants. Egg laying may last as long as a month and may
be as many as 500 eggs on a single plant. After four to 10 days
depending on temperature, eggs hatch. The cooler the air temperature
the longer it will take for hatching. The hatchlings are tiny larva.
The larva grow in four progressive stages (instars), eating the
host foliage all the while. Depending on temperature, the larval
stages will last two to five weeks or when about 400 degree days
have accumulated. Then, the larva will go into the ground, three
to six inch deep, and pupate. The Pupa last for five to 10 days
after which new adults emerge. The emerged adult travels around,
looks for mates, and after seven to 10 days begins to lay eggs.
The full cycle takes five to eight weeks per generation. Usually,
there are two generations per season in Nebraska, early to mid June
and early to mid August. In cooler States, less than two generations
may occur. At the end of the season, remaining larva move into the
soil and overwinter as pupa.
Damage
CPB
larvae are the most damaging form but adults also feed on the foliage.
Due to feeding, leaflets have holes of varying sizes usually starting
around the margins. The leaf blades are eaten often leaving a skeleton
of veins and petioles behind. This results in defoliation. Defoliation
threshold levels are reported as 25% before tuber bulking, 10% during
the first half of bulking, four to six weeks and 25% after bulking.
Vine damage results in yield loss due to loss of foliage to support
tuber growth and mis-shaping of tubers is also possible. Severe
damage may result in plant stunting as well.
Scouting
Monitoring
the edges of fields, nearby gardens and surrounding weeds especially
nightshades will give warning when CPB are entering the field. Since
they are poor fliers, CPB enter a field along the edge usually at
a corner that is closest to the previos year’s crop. Economic threshold
levels have been reported.
Table.
Economic thresholds for Colorado potato beetle based on market price
and cost of control.
Crop
Value Control Cost
| $/cwt |
$5/acre |
$10/acre |
$15/acre |
| 2.00 |
2-4* |
4-9 |
5-14 |
| 4.00 |
2 |
2-4 |
3-7 |
| 6.00 |
2 |
1-3 |
2-4 |
|
*
Values are numbers of larval-infested plants per 100 feet of row.
Do not apply products unless the average infestation level is greater
than these values. Bechinski,
E.J., Sandvol, L.E., and Stoltz, R.L. 1994. Integrated Pest Management
Guide to Colorado Potato Beetles. Univ Idaho Coop Ext Circ #757.
Management
Mechanical
The
trench method is a semi-successful mechanical control of CPB mostly
applicable to small acreage. In areas where close rotations are
used or potatoes are grown adjacent to last years potato fields,
this method may help to alleviate first generation CPB damage. A
trench is plowed between overwintering sites and this year's potato
field. The trench should be at least 12 inches deep with sides sloping
45 to 90o. Emerging beetles walk to find their early season hosts
and become trapped in the trench. A fine coating of dirt on the
plastic prevents the beetles from being able to get out of the trench.
Note some CPB adults may fly over the trench; however, this method
can be surprisingly effective at trapping beetles.
Using
a flamer is another mechanical method for controlling CPB. Its best
fit is also in areas where trap crops are used to congregate immigrating
beetles for easier control. Flaming is used on plants that are up
to 8 inches tall with little adverse effects on them. Larger plants
will provide more protection for the beetles and effectiveness is
reduced. Flaming involves using two propane-powered flame jets that
are directed in an offset manner toward the row. A single unit can
be bought for under $3,o00 or constructed. Propane usage is about
3½ to 5½ gallons per acre. Speed is also an advantage. As might
be suspected, you don't want to go too slow with this type of machine.
A speed of 4 to 6 mph is recommended. Control with the flamer should
be done on a clear warm day so the beetles are up on the plants.
Data for early-season control have been as high as 90% with this
method.
Moyer,
D.D. 1992. Fabrication and operation of a propane flamer for Colorado
potato beetle control. Cornell Coop Ext Bull. pp. 7.
Olkowski,
W., Saiki, N. and Daar, S. 1992. IPM options for Colorado potato
beetle. The IPM Practitioner 14:1-21.
Biological
Several
common beneficial insects, predators, are good agents for devouring
CPB eggs and even attacking larva; see lady beetles and others.
Since CPB overwinter in potato fields, rotating crops is key and
the following season’s potato fields should be planted a few miles
away from the previous season’s potato fields. Since CPB will develop
on weeds in the same family as potato, controlling these especially
the various night shades is an important part of the management
program. Bt products such as Dipel have been used to kill young
CPB larvae but timing is crucial for efficacy. In the 1990s, potato
clones began to be engineered (GMO) with Bt genes to protect plants
from CPB. This technology is very successful in doing that but due
to uninformed political pressure this GMO technology has been slowed
on potato although common in corn and soybean.
Chemical
Most
systemic insecticides applied at planting will kill the CPB's first
generation. Most foliar insecticides will take care of later invasions
by CPB. The key is not to let the population de-synchronize. If
this happens, weekly applications of foliar insecticides could be
needed. The best application stage is on the young larvae, first
and second instar, because they have most of their eating ahead
of them. Older larvae have less eating to do and also are less sensitive.
Killing adults works but the result may be so/so because they may
have already laid eggs and you may be killing their predators such
as lady beetles as well. These predators would devour the CPB egg
masses. Since CPB enter fields along the edges and gradually move
inwardly, with good scouting, often only the field’s perimeter needs
to be treated.
It
is very important to note that the CPB is highly capable to develop
resistance to insecticide products. On the East Coast and Great
Lakes region, the CPB has developed resistance to combinations of
products, multiple-resistance or “super” bugs. Therefore, management
needs to be an integrated program of biological and chemical methods,
at least until the consumer market allows resistant potato lines
to return.
Home
Gardens
To
control CPB in garden potatoes as well as peppers and tomatoes,
apply carbaryl (Sevin) to plants when the adults or young larvae
are present. But be aware not to treat around flower areas to avoid
killing honey bees needed for pollination.
Quick
Review
Appearance:
- Adult
- round with yellow and black stripping, 1/2 inch long
- Larva
- slug-like, pinkish with two rows of black dots on each side,
four instars from 1/10 to 1/2 inch
- Egg
- small yellow to orange in masses under leaflets
Life
Cycle:
- Overwinters
as an adult in soil of previous year’s crop
- One
to two generations which can de-synchronize easily
- Can
appear from April to September
Damage:
Defoliation;
tuber yield loss
Management:
- Crop
Rotation
- Beneficial
Insects
- Systemic
Insecticides
- Foliar
Insecticides but beware of resistance
- Bt
products
- GMO
potato engineered with Bt gene
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