During the summer season, bean plants may become unhealthy looking, with a mottled or stippled pattern of tiny whitish speckled spots, short streaks or short lines on the upper surface their leaves.
These affected leaved may eventually turn yellow, then brown, and become dry and shriveled.
This is damage caused by a tiny sap-sucking pest, the two-spotted spider mite, also known as the red spider mite (Tetranychus urticae), which is barely visible to the naked eye, being only 0.3-0.5mm long, that live on the underside of leaves.
Feeding damage caused by spider mites is first observed on the lower part of a plant and gradually moves upward as the mite population increases in numbers.
Spider mites will usually feed exclusively on one plant until it starts to weaken and decline, and then they will move on to a new plant.
A common characteristic of spider mite infestation is the presence of fine silk webbing similar to spider webs that may appear on new growth and between leaves but may cover whole leaves. These webs are used to line feeding areas to protect the mites, and to help them to be dispersed by the wind to move to new plants.
Heavy infestations of two‐spotted spider mites can cause severe crop damage and seriously weaken plants, causing defoliation and reductions in crop quality and yields.
Spider mites prefer a dry environment, so rainy weather helps to keep population levels in check.
How to Control Red Spider Mites
There are many safe and environmentally friendly ways to prevent and control spider mites, for more detailed information see the article – The Best Ways to Control Two-Spotted Spider Mites
To offer a quick run-down on the most popular methods, here’s a short list:
- Hosing – some gardeners simply hose the pests off the plants, which reduces their populations significantly.
- Horticultural oils and insecticidal soaps – are organically approved non-toxic contact sprays that smother the pests so they can’t breathe, and only work when sprayed directly on a pest to cover its body.
- Wettable sulphur – is an organically approved miticide used to eradicate pest mites and is also used as a fungicide. This colloidal form of elemental sulphur is a fine yellow-brown powder which dissolves in water and is designed to be sprayed. Regular agricultural sulphur which is used as a soil amendment, is not the same, as it can’t be dissolved in water and can’t be sprayed. Use the right one!
- Victoria State Government, Agriculture Victoria, Two-spotted mite, <https://agriculture.vic.gov.au/biosecurity/pest-insects-and-mites/priority-pest-insects-and-mites/twospotted-mite>
- Clemson University Cooperative Extension Service, BEAN & SOUTHERN PEA INSECT PESTS, Factsheet HGIC 2201, Updated: May 5, 2021. <https://hgic.clemson.edu/factsheet/bean-southern-pea-insect-pests/>
- University of Florida IFAS Extension Sarasota County, Edible Gardening Series: Question of the Week – spider mites by Carol Wyatt-Evens and Sarah Bostick <https://blogs.ifas.ufl.edu/sarasotaco/2021/04/08/edible-gardening-series-question-of-the-week-spider-mites/>
- PlantVillage, Beans, <https://plantvillage.psu.edu/topics/bean/infos>
- UNIVERSITY of NEBRASKA–LINCOLN, Nebraska Extension, Institute of Agriculture and Natural Resources, Cropwatch, Insects: Bean Leaf Beetle and Mites, https://cropwatch.unl.edu/soybean-management/insects-bean-leaf-beetle