Genista: a Botanical Puzzle

Source: Nicholson, Rachel. “Genista: a Botanical Puzzle.” Sussex Botanical Recording Society Newsletter, no. 60 (May 2005). http://sussexflora.org.uk/wp-content/uploads/2016/03/Newsletter_May_2005.pdf.

 

Have you ever wondered why whole plants of Genista tinctoria (Dyer’s Greenweed) sometimes produce no flowers? A chance remark set off an investigation into this phenomenon.

On examination of blind plants, shoots and leaves were found to be stunted, distorted, brown and with piercing marks on them. Many of the buds were brown. This was typical of the sucking damage done by mirid bugs. Invertebrates seen on the plants were noted and the most common was found to be one of these bugs, later determined to be Heterocordylus genistae. This active, black hemipterous bug, half a centimetre long, was seen on populations of Genista tinctoria at nine sites in E. Sussex. It is also said to occur on Cytisus scoparius (Broom).

Mirid bugs feed by injecting saliva into parts of plants which are actively growing, that is the young flowers or fruit and the growing tips of shoots, as well as young leaves (Strong et al. 1984, Hodgkinson & Hughes 1982). The saliva acts as an enzyme, breaking down the starch in the sap. This causes the browning of the tissues, which may then dry up (Miller 1956).

Heterocordylus genistae females lay their eggs three or four at a time, at right angles with the ivory-coloured tops exposed, in slits in the lower stems of the shoots. This position for the eggs, which will take nine months over winter to mature, helps to protect them from desiccation, mowing or predators. When the new buds begin to shoot, the nymphs are in exactly the right place to move up the stem and start feeding. It also explains how the insect can survive the annual cutting of the plant, formerly for use as a dye plant, nowadays as a component of a hayfield or verge. The nymphs hatch in late May or early June (Southwood & Leston 1954) and it is these which cause the early damage, preventing flower formation. They are adult by mid-June and can then fly to a new food source, for example the flowers on neighbouring undamaged bushes. The males die before the females, which lay their eggs in mid-July and may survive until August.

This almost complete dependence of an insect on a single host plant brings up the question of its future existence. We worry about what will happen to several moths and butterflies if ragwort is eliminated, but perhaps we should also think about what would happen to this, admittedly less charismatic, species, if Genista tinctoria, already rapidly disappearing, goes for good.

References

Hodgkinson, I.D. & Hughes, M.K. (1982) Insect Herbivory. Chapman & Hall.

Miller, N.C.E. (1956). The biology of the Hemiptera. Leonard Hill (Books) Ltd.

Southwood, T.R.E. & Leston, D. (1959). Land and water bugs of the British Isles. Frederick Warne & Co. Ltd.

Strong, D.R, Lawton, J.H. & Southwood, R (1984) Insects on plants. Blackwell Scientific Publications.