Useful information about cutworms

The damaging part of the life cycle of cutworm is the caterpillar-like larvae that live just below the soil surface. It is very easy to identify, as the larvae will curl into the shape of a ‘C” when hiding in the soil during the day. Typical feeding damage is a cleanly severed seedling at the soil surface, where the larvae have eaten through the stem.

Figure 1 a) Dorsal view of the larva of a black cutworm. b) Lateral view of the larva of a black cutworm Agrotis ipsilon (Hufnagel).

The adult moths are only active at night, when most growers do not spray crop protection agents, so they are rarely killed by pesticide applications in the canopy. The adult moth lays eggs on the soil surface, near to the emerging seedling, so that there is an immediate source of food for the young larvae. The moth lives about 3 weeks, during which time females could lay up to 2,000 eggs. Female moths can smell wet soil, from drip-irrigated crops and will lay eggs here because they are more likely to survive.

The cutworm larvae will spend most of its life in the soil near the plant stem. The larvae in the soil are very vulnerable to insect-killing fungi such as Metarhizium anisopliae (e.g. Real Metarhizium 69) and are an ideal place to target the pest. Depending on the soil temperature and availability of a host plant nearby, the cutworm can take from 34 to 114 days in the soil before it pupates.

Agronomic interventions

  • Early detection and intervention. Since adult moths may lay 2,000 eggs each, it is important to anticipate whether an invasion is possible and whether a second or third generation is developing within the crop. Action needs to be taken to prevent the second generation, which will have a greater egg-laying capacity and the potential to cause more damage than the first generation. Therefore even hand-collecting the larvae of the first generation will make an important contribution to crop protection. Each larvae collected, is one less egg-laying adult – which could lay up to 2,000 more eggs.
  • Use pheromone traps to detect the adult moth flights and take timely action to reduce egg laying and larval development. White and yellow traps are more effective than green traps. Use one trap per 3 hectares and treat the data as if it ‘belonged’ to all crops in that area – since the adults are highly mobile. A general indication of presence or absence and whether the levels are increasing or decreasing is sufficient to decide on the pest control treatments required.
  • Bait traps can detect the distribution of the larvae hiding in the soil – before the seedlings emerge. As soon as seedlings have emerged, the bait traps are less attractive (and less effective). The bait traps, will indicate, before damage occurs – where the pest is hiding in the soil. It consists of a container sunk into the soil, so that the upper lip is at the same level as the soil surface. The containers are baited with fresh plant material or bran to attract the cutworm and vermiculite can be used above the bait to provide some shelter from the light for hiding cutworms.
  • Use of non-host cover crops will not only improve the general soil health status of the soil but also help reduce pest, disease and weed pressure while an area of land is not in cropping. It is important to ensure that the intended cover crops are not hosts for cutworm to feed on. This will reduce the laying of eggs. Moths are more likely to lay eggs if host plants are present. The introduction of cover crops will also stimulate the presence of beneficial insects to naturally control all life stages of the pest
  • Weed control: Young weeds can be an important food source for cutworms, so good weed management will reduce the build-up of this pest. It is easier to get good coverage of the soil surface with a crop protection agent, if it is weed free.
  • Avoid wet fields or those, which have been flooded. Take preventive action to avoid this happening. This may mean the cutworm is worse in fields that are furrow irrigated rather than drip irrigated. Cutworm tends to be higher in such situations. Cutworm is sometimes called the ‘overflow worm’ because of this tendency.
  • Netting or row covers may reduce the egg laying activity of both cutworm and bean seed fly; but this is an expensive technology.
  • Destroy crop residues after harvest to reduce further generations form continued egg-laying.Crop residues can be mulched and utilised in compost.

Rotation is not a solution

Cutworm attacks so many different crops, that rotation is not a useful control for this pest.

Chemical interventions

The susceptible period for cutworm attack is quite long (3 – 4 weeks) so a persistent, surface applied chemical is needed. Chlorpyrifos fulfils this role in some countries where its use is permitted. If the label recommendations are followed, there should not be a problem with pesticide residues. However, not all customers permit chlorpyrifos to be used on export crops. Check with customers before use.

Application of systemic neonicotinoids to seeds can provide some protection to seeds and seedlings. However, many customers have restricted the use of any neonicotinoids, because of possible harmful effect on bees – so this may not be a possible choice for the grower.

Other broad-spectrum chemicals which are frequently used for cutworm control includes lambda-cyhalothrin. It has a contact action, so the cutworms must have emerged from the soil and begun feeding for the pesticide to come into contact with the chemical. However, pyrethroids will also kill parasitic wasps for cutworm and other pests for up to 10 weeks after the application of a single spray.

Biological interventions

Foliar applied bio-pesticides including entomo-pathogenic fungi and bacteria can be used in a management program to target the adult moths and eggs. Soil applied entomo-pathogenic fungi such as Real Metarhizium 69 will become more important in controlling the larvae and pupae life stage since soil application of broad spectrum chemicals will kill naturally occurring beneficial insects which parasitize or prey on cutworm.

The entomo-pathogenic fungi require contact on the pest to work. This can be achieved if a heavy irrigation is applied before sunset to bring the cutworm larvae up out of the soil – before the spray is applied. Entomo-pathogenic fungi can also be applied through the irrigation as the adult moth is more likely to lay her eggs on a soil surface that has moisture and is close to a food source.

Natural enemies of cutworm

According to published data, many wasps are known to attack cutworm: Apanteles marginiventris (Cresson), Microplitis feltiae Muesebeck, Microplitis kewleyi Muesebeck, Meteorus autographae Muesebeck, Meterorus leviventris (Wesmael) (all Hymenoptera: Braconidae); Campoletis argenti- frons (Cresson), Campoletis flavicincta (Ashmead), Hyposoter annulipes (Cresson), and Ophion flavidus Brulle (all Hymenoptera: Ichneumonidae).

Larvae parasitized by Meteorus leviventris (Wesmael) consume about 24% less foliage and cut about 36% fewer seedlings, so considerable benefit is derived from parasitism in addition to the eventual death of the host larva.

Other parasitoids known from black cutworm include flies often associated with other ground-dwelling noctuids, including Archytas cirphis Curran, Bonnetia comta (Fallen), Carcelia formosa (Aldrich and Webber), Chaetogaedia monticola (Bigot), Eucelatoria armigera (Coquillett), Euphorocera claripennis (Macquart), Gonia longipulvilli Tothill, G. sequax Williston, Lespesia archip- pivora (Riley), Madremyia saundersii (Williston), Sisyropa eudryae (Townsend), and Tachinomyia panaetius (Walker) (all Diptera: Tachinidae).

Predatory ground-dwelling insects such as ground beetles (Coleoptera: Carabidae) apparently consume numerous larvae.

It is important to seriously consider non-chemical interventions as the number of pesticide products are being reduced and the chance of residence build-up therefore increases.

Credit: Extract from IPM-Legumes-Manual, Louise Labuschagne