Stinkbugs are emerging pests in cotton. In conventional cotton the use of broad-spectrum insecticides to kill Helicoverpa spp. effectively controlled the stinkbugs, but with the introduction of transgenic Bt cotton the use of broad-spectrum insecticides to control Helicoverpa spp. has been reduced. Further reduction of insecticides is expected with the increasing adoption of Bollgard II and IPM will aggravate the stinkbug problem. Little was known about their damage, thresholds, IPM tools, etc. This project addressed these issues.

In Australia there are six different types of stinkbugs- green vegetable bug (GVB), Nezara viridula (Linnaeus), red banded shield bug (RBSB), Piezodorus hybneri (Gmelin), green stink bug (GSB), Plautia affinis (Dallas), brown stink bug (BSTB), Dictyotus caenosus (Westwood), harlequin bug (HRLQB), Tectocoris diopthalmus (Thunberg), cotton stainer bug (CSB), Dysdercus sidae (Montrouzier).

Stinkbugs move to cotton from wild winter or spring crop (early mungbean) hosts when these hosts dry off or are harvested after Christmas at boll setting stage and pass at least one generation causing considerable damage to cotton. All stinkbugs cause similar damage. Damage is characterised by black spots, warty growths inside boll walls, brown coloured lint and tight lock. The damage caused by stinkbugs cannot be distinguished from the damage cause by mirids at the boll stage. The most damaging stinkbug is GVB, causing damage 2, 3 and 4 times more than GSB, RBSB/CSB and HRLQB respectively. BSTB caused negligible damage. Fourth and fifth instar nymphs and adult GVB cause equivalent damage. Third instars cause half the damage caused by 4th and 5th instar nymphs and adults. First instars do not feed and 2nd instar nymphs cause negligible damage. Bolls up to 20 days old suffer significant damage from GVB but compared to older bolls, the preferred age is 10 days or less. Bolls up to 7 days can shed due to GVB feeding. Bolls older than 25 days suffer very negligible damage and therefore do not need protection at that stage.

The most efficient method to monitor stinkbug is beat sheet sampling. In the field, distribution of stinkbugs is patchy; therefore thorough inspections at least once in a week throughout a crop are necessary. Stinkbugs are most visible during the early to mid morning when they move to the top of the crop to bask in the sun, making crop inspections easier at this time.

Once stinkbugs number reach the threshold level, control option should be selected in the light of the IPM strategy. The threshold for GVB is 1 bug (adult, 4th and 5th instar nymphs)/m with beat sheet or 0.5 bugs/m with visual counting. When calculating threshold, 3rd instar are equivalent to 0.5, and 1st or 2nd instars, clumped around the egg remnants, are equivalent to 1 4th or 5th instar nymph or adult. The thresholds for GSB, RBSB/CSB and HRLQB are 2, 3 and 4/m with beat sheet and 1, 1.5 and 2/m with visual counting respectively. As well as the insect threshold, a damage threshold can be used for management decision. US guidelines suggest a damage threshold of 20% damage to small bolls (14 days old). At least 100 bolls from a management unit should be selected randomly to assess damage and the presence of warts or stained lint deems a boll to be damaged.

For managing stinkbugs, soybean strip or bulk can be used as a trap crop. Since soybean is a preferred host of whitefly, it can be replaced with mungbean where whitefly is an issue. Since stinkbugs preferred podding stage of soybean/mungbean, the trap crop should be planted in such a way that they start podding in early January when stinkbugs move to cotton from wild hosts.

Salt mixture is an effective and profitable IPM option to manage stinkbugs. Salt at 10 g/L water mixed with reduced rate (1/2 to ¼ of full rate) chemical increase chemical efficacy by 40 % compared to low rate of chemical alone. Salt mixture increased palatability of the chemical. Mixing salt with chemicals should be approached cautiously. Chemicals that are registered for Helicoverpa, mites, whitefly and aphids should not be mixed at the low rate with salt if one of these pests is present in the field. The stinkbug spray at lower rate may have resistance implications for those pests. In terms of the IRMS, a low-rate application is counted the same as a full-rate application. If there is a maximum of three applications allowed then three low-rate applications is equivalent to three full-rate applications.