Green bean pests and how to control them

Green beans are attached by various pests and because effective pest control is such an important aspect of green bean production, the bean pests must be identified before the correct control measures can be applied. The safe period applicable to a specific pesticide is very important. This period allows the chemical to bio-degrade and makes the yield safe to eat. The registered pesticides have safe periods and dosages indicated on the labels and instruction manuals inside the containers. The safe period is the time elapsed after pesticide application when you are allowed to harvest the crop.

Let’s have a look what damages various green bean pests cause and how to control them:

Damage to seed

The bean weevil (Bruchus abtectus) is a dark brown small weevil about 3 mm long. Beans are infected in the lands where females lay her eggs inside the ripe pods after they split open.  The larvae bore into the seed and during storage the damage can spotted.  If control measures are not taken during the storage periods, the population will keep on increasing.

Controlling bean weevil. There are no pesticides registered to control bean weevil in the land. The only way to control it is during the storage period itself. An airtight store may be fumigated, otherwise the shelves, walls and floor may be sprays with a pesticide. The seed can also be treated in an airtight container before planting. If the seed is stored below 6°C (±43°F), no chemicals are required.

Bean pests that damage roots or stems

Rootnot nematode (Meloidogyne spp.)

Infested plants show signs of retarded growth in spite of adequate moisture and nutrients in the soil. Such plantings have thickenings and galls on their roots. Extensive damage by nematodes causes the plants to grow poorly and die before fruit.

How to control nematodes in green bean plantings: Treat the well-prepared, slightly damp soil with fumigant. After the application the soil should be rolled and left for two to three weeks before sowing.

Seed-bean maggot (Hylemyia cilicrura)

This pest causes poor emergence of seedlings. Closer examination will show that germination did take place, but that the seedlings died as a result of maggots boring into the young shoots. Damage can be extensive, especially in cool, damp soil rich in decaying organic matter. however, this pest occurs sporadically.

Control: Avoid the addition of un-decomposed organic material shortly before sowing seed. Control measures may be applied by thoroughly mixing damp seeds with an insecticide before sowing.

Cutworm (Euxoa spp.)

The worm is grey-brown in colour can can be found during the day about 20-50 mm below the soil surface. During its nocturnal activities it cuts the seedlings at or just below the soil surface. The adult moth is also active at night and lays her eggs on a great variety of plants. No eggs will, therefore, be laid on fallow land which is free of vegetation.

Control: Clean cultivation of the land about six weeks before sowing will prevent the moths from laying eggs and cutworms already present in the soil will die of starvation. Another control measure is to distribute a poison bait a day or two before sowing. Maize-meal or bran is mixed with an insecticide and water into a soft paste and sprinkled over the land in late afternoon. Four ready-mixed baits are also available.

Cutworm. This one is brown but most are grey-brown. They surface during the night and damage seedling stems.

Cutworm moth Noctuidae

Bean pests damage to leaves and stems

Aphids (Aphis craccifora)(A. fabae)

Black or dark-brown aphids occur in colonies on leaves. They feed on the sap of the leaves which then curl up.

Control: A full cover spray with one of the systemic or contact insecticides is recommended.

Spider mite (Tetranychus spp.)

These mites are small and reddish-brown. They are found on the lower surface of leaves where they feed on plant sap and spin fine silk threads and webs. With a heavy infestation they are visible to the naked eye. Small light-yellow specks appear on the upper surface of the leaves, which lose their healthy sheen and may turn yellow or brown.

Control: As spider mites mainly occur on the lower surface of leaves, a full cover spray with an insecticide is extremely important.

Twig wilter (Anoplocnemis curvipes)

The sucking damage caused by this insect results in the wilting of young growth. The bug is dark brown, about 25 mm in length, with well developed hind legs and sharp spines on either side of the thorax.

Control: no insecticide is registered for Twig wilter.

CMR-Beetle (Mylabris spp.)

These beetles are 20-35 mm long and have black bodies with broad yellow stripes on their wings. All aerial parts of the plants are damaged, particularly the flowers.

Control: As soon as the beetles are noticed they should be sprayed to reduce populations. The number of Twig wilter and chafer beetles will also be reduced.

Chafer beetles (Melolonthinae and Rutelinae)

These beetles remain in the soil during the day, but at night damage the leaves of plants. They are brown and about 10 mm long.

Control: Apply registered insecticides regularly to young growth when pests appear.

Damage to pods

Thrips (Taeniothrips sjostedti)

This insect is yellow when immature and dark brown when adult. It is extremely small, 1-2 mm max. It feeds on flowers and causes damage to the young developing pods. The surface of these pods are rough with silver spots. Damage of thrips can range between 10-36% of pod yield¹⁾Duff, John & Church, C.E. & Healey, Madaline & Senior, Lara. (2015). Thrips incidence in green beans and the degree of damage caused. Acta Horticulturae. 1105. 19-26., so control is very important.

Control: A full cover spray with an insecticide is essential. Repeat 10-14 days later if required.

Bollworm (Heliothis armigera)

The green or brownish caterpillar which has a clear yellow stripe on either side of its body, feeds inside the pod. All stages of the developing pods are attached. The moths are night fliers. Eggs are laid single on flowers or young pods which are then penetrated by the newly hatched larvae.

Control measures should be taken as soon as fruit-setting are complete and repeated 7-10 days later. A third application may be required soon after the first picking.

Semi-looper or Plusia looper (Trichoplusia orichalcea)

The caterpillar is bright green and feeding damage is done to the surface of the pods. The caterpillar has a characteristic way of looping its body when in motion.

Control: A full cover spray with an insecticide is recommended. Consult the

Plusia looper damage to leaves

Insecticides that can be used to control green bean pests

Below are some trade names and active ingredients that can be used on green beans. The trade names can change, but active ingredient does not. The list is not complete and changes every year. So this list is only provided as a guideline.

• biteranol EC: Baycor
• carbendazim SC: Bavistin, Delsene, Derosal
• chlorothalonil SC: Bravo, Chloroflo, Chloronil, Clortosip, Clortosip L, Kynothal, Mycoguard
• chlorothalonil WP: Bravo, Bactume 30 & Clortosip
• copper amonium carbonate SL: Copper Count N
• copper oxychloride WP: Blitox, Copper Oxychlor, Copper Oxcychloride, Cupravit Copper Spray, Koperchlor, Rust, Virikop
• cupric hydroxide WP: Cudrox, Fungura-OH, Hydrox, Kocide 101, Cupric Hydroxide
• cupric hydroxide WG: Kocide DF
• cupric hydroxide SC: Kocide 606
• cyproconazole SL: Alto, Atemi
• difenoconazole EC: Flare, Score
• hexaconazole SC: Anvil, Neptune
• mancozeb WP: Dithane M45, Macozeb, Sancozeb
• mancozeb WG: Penncozeb
• mancozeb DP: Dithane M45, Sancozeb
• maneb/zinc oxide SC: Trimangol
• myclobutanil EC: Systhane
• oxycarboxin EC: Plantvax
• procymidone SC: Sumisclex
• tebuconazole EW: Folicur
• tebuconazole EC: Folicur
• triforine EC: Denarin, Funginex, Saprol
• zineb WP: Zineb

The various abbreviations are described below:

• EC: emulsifiable concentrate
• SC: suspension concentrate
• WP: wettable powder
• EW: emulsion, oil in water
• SL: soluble concentrate
• WG: water dispensable granules
• DP: dusting powder