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SAND
CHAFER (adult) and WHITE GRUB (larva)
White
grubs, in general, refer to several species in the “May” or “June”
beetle family injuring primarily grass crops. These grubs are commonly
on a three-year cycle with the larval stage remaining the soil.
However, these do not seem to affect potatoes or corn. The annual
white grub that seems to affect potatoes is referred to as the “sand”
chafer (Strigoderma arboricola) because it is associated with sandy
ground. There are other chafers whose adults may be found in potato
fields, most notably the “rose” chafer (Macrodactylus spinosus)
and the “masked” chafer (Cyclocephalia sp.), but these do not seem
to produce the white grubs that affect potato tubers.
Description
 Adult
sand chafers are shiny black with rusty-red wing covers. They appear
bronze or copper from reflecting light when seen from above. Their
length is about three-eighths of an inch, similar in size and shape
to Japanese beetles and half the size of June beetles.
Eggs
are deposited in the ground and hatch as larvae commonly called
white grubs and these remain under the ground. White grubs are dirty
white and partially transparent. They are small, fat and worm-like,
and are about an inch long. Their head is brown and behind the head
near the front of the grub are six prominent legs.
Life
Cycle
The
sand chafer has an annual life cycle, going through one generation
per year, unlike that of the June beetles. Adults emerge from the
ground in early summer, latter half of June and early July. They
are highly attracted to light and will land on light-colored clothing,
get under clothes and tangle in one’s hair. Adults live about 11
to 31 days. Upon emerging from the ground, adults mate and eggs
are deposited in the ground. Peak egg laying time is July. Eggs
hatch after 11 to 25 days in August. Hatched from eggs are the larvae
or white grubs. These feed on potato tubers in August and September
depending on ground temperature. They overwinter deep in the soil
for five to 10 months and become active again as the ground warms
up in the spring then they pupate and transform into adults. Adults
are the only stage in the life cycle that is above ground; all other
stages are below ground.
Damage
 The
damaging stage of the life cycle is the larvae or white grub. The
overwintered grub feeds little before pupating; there have been
a few reports of grub feeding of seed-pieces. Damage occurs on the
tubers to be harvested late in the season. As the newly-hatched
grubs enlarge, feeding increases. Reports indicate that the worst
damage occurs after vine desiccation when done in late August. Vine
desiccation reduces ground shading and indirectly allows the ground
to warm up more. They move up in the soil due to warmth and encounter
the tubers. It has also been suggested that the white grubs feed
on the root hairs of the potato plant inflicting little damage but,
after vine desiccation, the hairs dry up and the grubs go to the
tubers to feed. Damage appears as large, shallow, irregular and
ridged gouges. Gouges are usually a quarter to a half inch deep
running along the surface of the tuber and not drilled into the
tuber as one would see with wireworm holes. White grub feeding ceases
as the soil cools and they move deep into the ground to overwinter.
[In
corn, white grubs of the sand chafer feed on the root system.]
Control
Sustainable
Agriculture --
White
grubs of the sand chafer affect corn as well as potatoes; large
populations are especially found in corn fields planted after soybeans.
Since both of these crops are commonly used in rotation with potatoes,
crop rotation has little effect in reducing sand chafer populations.
Sand chafer and white grub infestation seem to be worst in soils
with high organic matter and on ground treated with manures. Otherwise,
there is very little known on controlling this pest with cover crops
or other methods of sustainable agriculture. When possible, the
best is to vine desiccate as late as possible, mid-September and
later.
Chemical
Management --
As
with sustainable agriculture, there is little known about chemical
control of sand chafers and their white grubs. Retired Colorado
State Univ. entomologist, Dr. Stan Pilcher who identified the sand
chafer as a source of white grubs affecting potatoes in the mid-1990s,
found this pest to be tolerant to most available insecticides. As
of now, I know of no anti-insect product with sand chafer or white
grub on its label, although there are a few with rose chafer on
theirs. The most effective treatment so far has been using carbofuran
(Furadan). In 1999, in a conversation with Dr. Pilcher, he noted
that Furadon worked best in laboratory tests showing three to four
days of activity against the grub that phorate (Thimet) applied
in-furrow followed by Furadan application showed about a 20% reduction
of population.
A
good program used by some growers in recent years has been applying
Furadan by air on adults in early July followed by a late season
application of Furadan for white grubs. The late season application
should be between mid-August and mid-September, preferably shortly
after vine desiccation. NOTE, Furadan can NOT be applied within
14 days of harvest (14-day pre-harvest interval, PHI) and has a
14-day restricted entry interval (REI). Application should be with
a ground rig followed quickly by 0.1 to 0.2 inches of irrigation
water from a sprinkler to incorporate Furadan down a few inches
to the grubs. Furadan works best when incorporated at the end of
the season, and it readily moves in water, will leach and spread
in wet soil. NOTE, Furadan is NOT labeled for sprinkler irrigation
(chemigation) as it will accumulate on the center pivot and affect
birds that may wash in or drink from it. Anecdotal reports suggest
that imidacloprid (Tops MZ Gaucho, Genesis, Admire, Provado) applied
at planting in conjunction with a later Furadan application may
be effective in reducing white grub damage on harvested tubers.
This still needs to be verified.
Quick
Review
Appearance:
- Adults
- shiny bronze or copper, ¼-½ long
- Larvae
- white grubs, 1 inch long
Life
Cycle:
- Annual,
one generation
- Adults
- late June to early July
- Grub
- August to September (overwinters)
Damage:
- Adult
- none
- Grub
- gouges along tuber surface
Management:
- Crop
Rotation - no control after corn
- Desiccation
- after September 10
- Manure
- avoid application
- Chemical
--
- Admire/Genesis
etc - at planting against emerging adults/grubs
- Furadan
- early to mid-season against adults
- Furadan
- late season against white grub
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