Control Methods for Major Underground Pests of Potatoes

I. Major Pests

1. Mole cricket, also called lala cricket or earth dog. Both the adults (with fully developed wings) and nymphs (with incompletely developed wings) of the mole cricket cause damage to potatoes. It uses its mouthparts and large front claws (forelegs) to tear the underground stems or roots of potatoes into a tangled mass, causing the above-ground parts to wilt or die. It sometimes also bites the seed pieces, preventing the buds from growing and causing gaps in the stand. It tunnels through the soil, separating young roots from the soil and exposing them to air, causing water loss that affects seedling growth and can even kill the plants. In autumn, it bites the tubers, creating holes in them or making them susceptible to infection by rot fungi, causing decay.

Both the adults and nymphs of the mole cricket move up and down in the soil according to changes in soil temperature. During overwintering, they burrow down 1.2-1.6 m to build chambers and become dormant. In spring, as the soil temperature rises, they move back up to the 10 cm deep plow layer to cause damage. They stay underground during the day and come to the surface at night. When summer temperatures are high, they move down to about 20 cm deep, and in autumn they return to the plow layer to cause damage. Generally, mole crickets are most rampant in soils with relatively high organic matter content and low salinity and alkalinity.

2. White grub, also called soil silkworm, is the larva of the scarab beetle. In potato fields, it mainly damages underground tender roots, underground stems and tubers by biting and boring, with clean-cut wounds, cutting off the supply of nutrients and water to the above-ground stems and causing them to die. After tubers are bored into, their quality is lost or they become susceptible to rot. The adults (scarabs) also fly onto plants and bite the leaves.

Both the white grubs and their adults can overwinter and move vertically up and down in the soil. Adults overwinter below 40 cm underground and larvae below 90 cm underground, then rise to the plow layer at about 10 cm deep in spring. They prefer organic matter and like to live in horse and mule manure. Adults are active at night and hide in the soil during the day. Larvae have 3 pairs of thoracic legs, are plump, milky white, often curl into a horseshoe shape, and exhibit thanatosis (playing dead).

3. Click beetle larva, also called wireworm, is the larva of the click beetle. It causes damage as a larva, boring into seed pieces, roots and underground stems in spring. Although thicker roots or stems are rarely bitten through, the seedlings gradually wilt or die. In autumn, the larvae bore into tubers, forming a tunnel through the tuber flesh, reducing tuber quality and sometimes causing rot.

Both the adults and larvae of the click beetle can burrow into the soil below 60 cm to overwinter, leaving wormholes when they burrow in, and in spring they rise back to the plow layer through these holes. When the summer soil temperature exceeds 17°C, they gradually move downward; after the surface temperature drops in autumn, they enter the plow layer again to cause damage. Newly hatched larvae are white and change color as they grow. They are glossy, hard-bodied, 2-3 cm long and slender.

4. Cutworm, also called soil silkworm or root-cutting worm. The larvae cause the greatest damage. The adult is a type of noctuid moth, including the greasy cutworm and the yellow cutworm, among others. Cutworms mainly damage the seedlings of potatoes and other crops by biting through the seedlings near the soil surface, killing the entire plant, and often dragging the bitten-off seedlings into their burrows. Young larvae also bite the tender leaves, causing notches and holes in the leaves. They also bite tubers underground, and the holes they make are smaller than those made by white grubs.

Cutworm larvae are yellow-brown, dark brown or black-brown fleshy worms, generally 3-5 cm long. The greasy cutworm prefers damp and shaded environments, and pest numbers are high where field coverage is dense, weeds are plentiful and soil moisture is high; the yellow cutworm prefers dry environments, has low moisture requirements and fears heat in summer. Both adults are phototactic and attracted to sugar and honey.

II. Control Methods

The several underground pests mentioned above are all different from one another, yet they also share similarities. They all are active underground, so the control methods are broadly the same.

1. Deep plowing and deep harrowing in autumn. Destroy their overwintering environment, freeze to death large numbers of larvae, pupae and adults preparing to overwinter, reduce the overwintering population and lessen the damage in the following year.

2. Clean up the field. Remove weeds and debris from the field, field ridges, field edges, field borders and ditch banks, and carry them out of the field for disposal, in order to reduce the number of larvae and eggs.

3. Trap and kill adults. Use sugar-honey traps, black light lamps, fresh horse manure piles, grass bundles and the like to trap and kill adults that are respectively phototactic, attracted to sugar and honey, or attracted to horse manure, which can reduce adult egg-laying and lower larval numbers.

4. Chemical control. Use poisoned soil and granules: at sowing time, apply 3-4 kg of 1% trichlorfon powder per mu, mix evenly with 10 kg of fine soil, or use 1.5-2 kg of 3% carbofuran granules and Dafenglei, etc., and scatter them along the furrows to kill underground pests damaging the crop at the seedling stage. Or apply the above pesticides around the base of the seedlings during intertillage to kill the pests.

Root drenching: use a 1500-2000 times dilution of 40% phoxim to drench the roots at the seedling stage, 50-100 ml per plant.

Use poisoned bait: for small-area control, the above pesticides can also be mixed into roasted wheat bran, corn or chaff to make poisoned bait, which is scattered in the field in the evening.