Organic pest management: Sand Chafer, White Grub in potoates
Last year was the first year we dealt with Sand Chafer, or White Grub in our potato crop. The damage to potato crops from this pest can be devastating. White grubs, the larval stage of the Sand Chafer beetle lifecycle, feed on potatoes just as their about to be harvested in August and September.
Signs and symptoms:
Adults are the only above-ground part of this insects life cycle:
White grubs are large and easy to spot if they're in your potato patch. They are sort of horrendous-looking things:
Their damage is large bites, which create craters in the potatoes. They seem to prefer the flesh to the skin and can hollow out a full potato, leaving a ghost-like round shell where a lovely little potato once was:
White grubs go through one generation per year. Adults emerge from the soil in late June, early July. Once out, adults live 11-31 days, during which time they mat and lay their eggs in soil. Eggs hatch larva in August. Larva feed in August and September. Prior to vine desiccation, larva prefer to feed on vines and the fine root hairs on the vines. Damage to potatoes will be very minimal, if at all, during this phase. After vines desiccate however, larva will move to potatoes to feed. Thus, the damage phase of the life cycle is very predictable and monitorable, one redeeming quality of this pest.
As soon as the ground becomes cool and moist is fall, larva tunnel deeper than potato level to overwinter. They will pupate in spring. There have been some reported incidence of pupa feeding on newly planted seed pieces in spring, but this is not common and I have not seen it. Thank goodness.
Monitor starting in August- Be sure to dig potatoes throughout the late summer to check for larval damage. If damage is seen, you may have to proceed with one of the following control methods.
Mound potatoes late in the season- Larva move up to warmer soils where they find potatoes to feed on. Mounding potatoes late in season may reduce damage by cooling soil around the potatoes.
Harvest early, prior to or just after vine desiccation- Because damage happens late in the season, harvesting potatoes early may be the most foolproof way to reduce damage. Monitor for larva, watch the vine dry down, and harvest accordingly.
Keep soil moist in late season- Larva seem to not travel as well or be as abundant in moist soil. Watering in late season may reduce damage.
Plant away from grassy edges- White Grub is much more well known for the damage is does to turf and pasture grass roots. Plant your potatoes away from the grassy edges of your garden
Crop rotation- Usually a must for potatoes, presence of White Grub adds incentive to never plant potatoes where you grew them last year. Because they over-winter as larva in the same spot they fed the season before, 3 year rotation is necessary.
Row cover- Row cover may work if you are planting your potatoes in a spot where you have not seen larva before. Place it on at planting time to exclude females from laying their eggs by the potatoes.
However, unlike flying insects beetles could easily walk under the row cover to lay their eggs, so it may not exclude this pest entirely. Additionally, potatoes need to be mounded through the season, so row cover gets in the way of this. We grow our potatoes at such a scale as to make row cover an impractical way to solve this problem, but it could help on a smaller scale.