Blackfly on Beans: How to Manage Infestations

Urgent action

Blackfly on beans is usually the black bean aphid, a sap-sucking insect in the Aphis fabae species group that affects broad beans, French beans, runner beans and many other garden plants. [1] The main risk period runs from April to August, so bean plants need close checks through the main growing season. [1] Blackfly are especially associated with tender new growth, and dense clusters can gather around the top-most foliage of broad beans. [7]

Left unchecked, blackfly can multiply rapidly, and strong broad bean plants can become weakened with shrivelled flowers and weakened foliage. [7] On broad beans, blackfly often move first to the tips of the plants. [3] A visible infestation can develop from first appearance to a bad infestation in only a couple of days. [3] A severe attack can destroy flowers before they turn into beans. [3]

Ant activity can be an early warning sign because ants may be seen going up and down broad bean plants when blackfly are present. [3] Young pods can also be infected with blackfly. [2] Broad bean tips can become infested with blackfly. [2]

What to look for

Black bean aphid is described as a black sap-sucking insect. [1] The affected plant range is wide and includes broad beans, French beans, runner beans, nasturtium, dahlia, philadelphus, viburnum and euonymus. [1] The common name is black bean aphid, and the scientific name is the Aphis fabae species group. [1]

The pest is attracted to new and tender growth on a range of plants. [7] On broad beans, the top growth is a common place to find dense clusters. [7] The first beans may be nearly ready to pick at the same time blackfly begin appearing on broad beans. [3]

Blackfly suck sap from the plant. [7] Blackfly can suck out plant goodness and can destroy much of a crop if they are not brought under control. [3] Damage may include weakened foliage and shrivelled flowers. [7]

First response

Pinching out the tips is a practical response when blackfly are concentrating on broad bean tips. [8] Wiping down leaves is another natural method listed for reducing damage on fava beans affected by blackfly. [8] A water hose can knock back a heavy covering of blackfly on plants. [5] Smooshing blackfly by hand can also reduce numbers when many are covering a plant. [5]

A simple soap spray can be effective and eco-friendly when done properly. [4] Floating row covers can be used on young plants to block early infestations. [4] Companion planting is another method to consider for fava beans affected by blackfly. [8]

Dilute neem oil with dish soap has been used against large infestations. [6] Neem oil sprays should not be applied in direct sun. [6] Spraying twice a day with dilute neem oil and dish soap has been described as a response to large infestations. [6]

Step-by-step plan

Inspect the tips first. Blackfly head for the tips of broad beans first, so checking the growing tips gives gardeners an early place to focus. [3] The pest can move from first appearance to a bad infestation in only a couple of days, so repeated checks are important during the April-to-August timing window. [1][3]

Remove the worst clusters quickly. Pinching the tips can reduce blackfly damage on fava beans. [8] Hand removal can also be used by smooshing blackfly when many are covering the plant. [5] A water hose can knock blackfly back from an affected plant. [5]

Protect young plants early. Floating row covers on young plants can block early infestations. [4] This is most relevant before blackfly have already formed dense clusters on tender new growth. [7]

Use sprays carefully. A simple soap spray can be an effective eco-friendly option when used properly. [4] Dilute neem oil with dish soap has been used for large infestations, but it should not be sprayed in direct sun. [6]

Watch the flowers and pods. Blackfly can destroy flowers before they turn into beans. [3] Young pods can become infected with blackfly. [2] Shrivelled flowers and weakened foliage are signs associated with unchecked blackfly multiplication. [7]

Prevention habits

Broad bean plants that overwinter are said in gardening lore to be less likely to be attacked by blackfly than spring-sown beans, but this does not hold true every year. [7] Because blackfly timing runs from April to August, prevention should be treated as a seasonal routine rather than a one-time job. [1] Dense clusters may appear around the top-most foliage, so prevention should include regular checks of the top growth. [7]

Companion plants are worth considering as part of a natural blackfly plan for fava beans. [8] Floating row covers are useful on young plants before early infestations establish. [4] Tip pinching, leaf wiping and companion planting are listed as natural methods to reduce blackfly damage on fava beans. [8]

What to watch

The key watch period is April through August. [1] The key plants include broad beans, French beans and runner beans, along with nasturtium, dahlia, philadelphus, viburnum and euonymus. [1] The key warning signs are clusters on tender new growth, blackfly on broad bean tips, ant movement on plants, weakened foliage and shrivelled flowers. [3][7]

The next priority is speed, because a small appearance can become a bad infestation in only a couple of days. [3] Gardeners who act at the tip stage can use pinching, wiping, hosing, hand removal, row covers or carefully applied sprays before flowers and young pods are badly affected. [4][5][6][8]