Butterfly bush (Buddleia davidii) is a perennial flowering shrub prized for its ability to attract butterflies and hummingbirds to its 6 to 12-inch long bloom spikes. (See reference 1) The white blooming variety also has a strong floral scent as evening falls to attract moths. While butterfly bushes aren't subject to many diseases or pests, and often grow back from the root despite tough winters, (See reference 1 and 2) its leaves may yellow when subject to undue stress.
Spider mites can infest a butterfly bush during the warm months, especially if water is scarce. (See reference 2) The damage may look like tiny yellow prickles on the tops of leaves that spread, eventually discoloring most of the leaf. Check the undersides of the leaves. Spider mites look like tiny black or red pinpricks and move around when its warm. There may also be webbing covering the colonies of spider mites.
Poor nutrition can cause the leaves of butterfly bush to discolor. Lack of iron, for example, may cause parts of the leaf to yellow while the veins stay green. Lack of sufficient light may cause the lower leaves to yellow and drop. Some cultivars of butterfly bush may die back to the roots during a harsh winter. Leaves may yellow before they drop. Before new foliage arrives in the spring, check to see if the stalks are still alive by scratching the outer bark. If the inner bark is hard and dry, the stalks can be trimmed at ground level.
Depending on how much of the butterfly bush is covered in yellow leaves, the plant may become weakened from inability to photosynthesize and produce its food. If it is producing new leaves in an attempt to recover, and these leaves are green, then the butterfly bush is recovering. If there are no new leaves, it is likely that the bush is going dormant. If the new leaves are also yellow, then it is having a hard time recovering and could use all the help it can get. Once the problem is diagnosed and treated, it may take a few days to several weeks to see recovery.
If you can't diagnose the source of the leaf yellowing, take a branch with several affected leaves to a local garden center for identification. Avoid spraying butterfly bushes with insecticides near or during the blooming cycle. (See reference 2) There are a many beneficial insects, butterflies, bees, moths and other insects that flock to blooming butterfly bushes. Chemicals that kill spider mites often do a better job of killing the beneficial insects!
If the butterfly bush is malnourished, add compost or organic fertilizer to give the butterfly bush a boost, especially during its prolific blooming season. Improper nutrient balance in the soil, excess heat, a lack of rain or other factors may lock up nutrition already present in the soil, so adding intense chemical fertilizers may not solve the problem. Chemical fertilizers may also be harsh enough to harm beneficial insects during vulnerable stages of their life cycle. Try the gentler solutions before bringing in the 'big guns'.