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.
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.
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.
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.]
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.
Adults - shiny bronze or copper, ¼-½ long
Larvae - white grubs, 1 inch long
Annual, one generation
Adults - late June to early July
Grub - August to September (overwinters)
Adult - none
Grub - gouges along tuber surface
Crop Rotation - no control after corn
Desiccation - after September 10
Manure - avoid application
Admire/Genesis etc - at planting against emerging adults/grubs
Furadan - early to mid-season against adults
Furadan - late season against white grub