Black bean aphid – Aphis fabae
Black bean aphids overwinter on the spindle tree, migrating to bean and sugar beet crops in the summer. Egg counts may be conducted to assess the risk of infection each year. Damage is mainly caused by aphid populations which colonise plants prior to flowering. These colonies result from the primary migration from overwintering hosts in mid May to mid June. Aphid populations initiated after flowering cause little damage. These are often secondary migrants, usually from sugar beet crops. Spraying is considered to be economic if 5 % of stems are infected by primary migrant on the south west headland of the field by mid June.
Late sown crops are generally at greater risk as the aphid migration is more likely to coincide with an earlier developmental stage of the crop. Control is achieved with pirimicarb ('Aphox'). Low rates of pirimicarb are almost as effective as the full recommended rate, especially if aphid predators are present in large numbers. The presence of mummified aphids is a good indicator of predator presence and can be used as an incentive to reduce rates.
Bruchid beetle – Bruchus rufimanus
Bruchid beetles belong to a group of insects originating in Africa and the Mediterranean area. They were probably bought to the U.K. in imported seed where they pupate. Adults appear in spring or summer, usually when temperatures exceed 20 degrees C for a number of days and lay their eggs on the flowers, young pods or seed of bean crops, boring into the young pod and seed. Larvae pupate and hatch into adults which bore out of the seed leaving characteristic exit holes which ruin their appearance. Bruchid beetle damage can reduce seed germination, especially if establishment conditions are poor as solutes leak from the endosperm through the exit holes, encouraging soil fungi and damping off diseases. However, the main effect of bruchid beetle damage is to reduce the quality of beans grown for human consumption. These are mainly spring beans, especially the tannin free varieties.
Control is difficult since the adults are present over a long period of crop growth and insecticides are not persistent. Applications of pyrethroids during flowering are most effective, the first applied when adult presence is first detected (usually early May) followed by a second at late flower to early podding.
Stem nematode – Dictylenchus dipsaci
Stem nematode lives in the plant tissue and seeds and can withstand severe desiccation. It also lives in the soil and can survive several years without a host crop. Hosts include beans and oats but will also infect sugar beet, peas, maize, strawberries and many weeds including red dead nettle and knotgrass. It is recognised that the Giant race is the one which affects beans most.
It is best to avoid cropping pulses too closely together in the rotation to help prevent infection, although if this coincides with a year of high rainfall particularly in the spring then chances of infection are increased.
Infected plants become swollen or distorted, later becoming dark brown in colour. This dark colouration usually starts at the stem base and stops at a leaf node. Infestations in the podding area result in infested seed. Heavily infested seeds have a blemished, darkened and cracked seed coat. One seed can carry 1500 to 2000 nematodes in more severe situations and is capable of infesting clean soil.
A seed test exists for stem nematode. Results are indicated as either present or absent although more detailed scoring is available from PGRO and NIAB. Uninfected land should not be planted with seed at any level of infection, low infection in dry years will not normally cause further issue, although this is not a risk that many growers should take.
Where infection has been notably present in the field in the past then this may prevent pulse from being grown on those fields for 6 or 12 years.
Pea and Bean weevil – Sitona lineatus
Pea and bean weevil is a grey brown weevil with characteristic snout. It feeds on leaves, producing u shaped notches but it is its larvae that produce most of the yield reductions in beans. Eggs are laid on the soil and the larvae hatch, burying into the soil to feed on the roots. Pea and bean weevil can dramatically reduce root nodule formation and thus nitrogen fixation.
Control of pea and bean weevil centres around the adults. Pyrethroid ('Hallmark', 'Dovetail' and 'Decis', cypermethrin (various) 'Contest', 'Fury') insecticides are applied in the spring once leaf notches become apparent. A second application 3 weeks after the first is often beneficial if infestations are high as this controls the adults hatching from larvae, laid as eggs by the first migrators into the crop.
Slugs are often prevalent in heavy clays where winter beans are generally cultivated, thriving in the wet conditions that often prevail during crop establishment, especially in the cloddy seedbeds produced when crops are ploughed in.
Slugs are controlled via ingestion of various pelleted insecticides. Slug death depends on the amount of insecticide ingested. Thus the slugs must feed continuously on the pellet in order to maximise their effect. Some pellets contain attractants or sugars to increase palatability which will increase the amount of insecticide eaten by the slug.
The insecticides used to control slugs differ in their mode of action and tend to perform differently under varying environmental conditions.