Know your crop pests: cotton stainer

Cotton stainers resemble assassin bugs and may be up to 2cm in length, with males being smaller than females.

A cotton stainer feeding on a seed.
Photo: Paul Donovan
    • Scientific name: Dysdercus spp.
  • Family: Pyrrhocoridae.
  • Distribution: A global problem, with a large number of species endemic to South Africa.

Their colours and patterns vary according to the species – tan, red, black and white usually feature. Adults are strong fliers and capable of dispersing over considerable distances.

With the exception of one species, all are pests of cotton. They also feed on hibiscus, okra, sorghum and millet. Adults and young occur in large groups and a cotton boll can appear red with the number of individuals swarming all over it.

Both adults and nymphs feed primarily on the seeds, sucking out the juice with the aid of their piercing mouth parts. These seeds will not ripen. The insects are also vectors for fungal infections that stain the lint yellow or brown.

Adults tend to appear as the boll first begins to open. They then hide inside and go unnoticed until processing. In the meantime, they taint the cotton and release an unpleasant odour.

Rapid Reproduction
Depending on the species, the female lays 100 to 130 small pale eggs in the soil, among plant debris or in the cotton plant. Incubation can take from a few days to around two weeks. The emerging nymphs are blood-red, gradually changing in colouration through the five moults that eventually lead to adulthood in 40 to 50 days. It is not until the final moult that the wings appear.

A variety of Control measures
Chemical: Control the pest with synthetic pyrethroids, carbamates such as carbaryl, or organophosphates such as dimethoate. These should be used only when the threshold level exceeds three cotton-stainers per square metre. Avoid killing natural predators. A new growth regulator – 2-methyl-4-phenyl-6-parachlorphenoxy-5-6-dihydropyrane (MPCD) – has undergone trials in some areas. Although it is early days yet, the results look promising.

Natural predators: Natural control can be highly effective in managing cotton stainers. One of the most reliable predators is the cotton stainer assassin bugs Phonoctonus spp. It mimics the colouration of the cotton stainer, so take care not to kill this valuable ally when treating cotton plants. They can easily be told apart from their behaviour: cotton stainers attack plants, whereas assassin bugs are only carnivorous. Adults may also be parasitised by the Tachinid flies Alophora spp, and spiders will also prey on them.

Other methods: 
Bait traps are widely used in many countries. These are intended to draw the cotton stainers away from the cotton plants, and usually take the form of piles of peeled sugarcane or cotton seeds. Manual removal of the bugs by flicking them in a bowl of waterand then disposing of them can prove effective too.