Green vegetable bug (GVB) in late cotton

There have been several reports of GVB occurring in late cotton crops. This has led to some confusion in the industry as to whether GVB needs to be controlled at this late stage in cotton. Research done by DEEDI entomologists has shown that younger bolls are more susceptible to lint damage caused by GVB feeding. These factors need to be considered when making management decisions regarding the control of GVB.

When are bolls susceptible to GVB damage?

Bolls aged 20 days and over (these bolls are hard and about ¾ inches in size) do not incur significant lint damage (brown coloured lint or tight lock) even if fed on by GVB. This means that even if GVB are present in the field, if the youngest bolls are older than 20 days GVB control is not warranted.

Bolls younger than 7 days may shed as a result of feeding by GVB.

Thresholds for GVB

Action thresholds for GVB in cotton are

1 bug per metre when sampling with a beatsheet
0.5 bug per metre for visual sampling

Parasitism by Trichopoda – do parasitised GVB cause any damage?

Research shows that a GVB parasitised by Trichopoda can continue to cause damage for up to 2 weeks after being parasitised.

Trichopoda giacomelli is an important natural enemy of GVB. This fly parasitises mainly adult GVB and in some cases late instar nymphs. Female Trichopoda lay eggs on GVB and when the larvae hatch, they burrow into the bug and feed on its internal organs and body fluids. Within 2 weeks, final-instar Trichopoda larva emerges from the GVB and pupates in the soil.

Although Trichopoda does not kill the GVB immediately and damage can still occur, they do contribute to GVB mortality and help reduce the size of following GVB generations, potentially reducing the risk offuture damage.

Parasitised GVB adult

Which life stage of GVB causes maximum damage?

Late instars nymphs (4^th and 5^th instars) cause similar damage as the adults. However third instars will cause about half the damage as an adult while 1^st and 2^nd instars cause up to one third the damage.

Other factors to consider when controlling GVB in late cotton

Good spray coverage is a prerequisite to successful management of GVB. Unfortunately there are no selective insecticides available for the control of GVB. Hence any chemical used to control GVB will disrupt populations of beneficial insects. This is an important consideration especially when silverleaf whitely or aphids are also present in the field. Disrupting the natural enemies of these pests can lead to bigger problems such as lint contamination.

Article by Moazzem Khan