Beware of Apple Dimpling bugs

There have been several reports of high numbers of apple dimpling bug (ADB) in early squaring cotton throughout the major cotton growing valleys. Also known as the yellow mirid, ADB adults (about 3mm long) are about one third the size of green mirid adults. They are yellow-green, have dark spines on the legs and hairy wings that are folded flat on the back. Apple dimpling bugs are capable of moving quickly and have a distinctive apple smell when squashed.

Apple dimpling bug adult

Pest or predator?

ADB are well known as predators of helicoverpa eggs and mites. However, they are also plant feeders and on young cotton will feed on small ‘pin’ squares which results in shedding of the squares. Damage at seedling stage is unlikely to affect yield but will delay maturity by 4-5 days.

Damaged (left) and undamaged squares © Mary Whitehouse CSIRO

When do they need controlling?

Dr Moazzem Khan (DEEDI) has studied the responses of cotton to ADB. Based on his findings – current recommended economic thresholds for ADB is 10 bugs per metre row of cotton along with 50% fruit retention at the squaring stage.

Monitoring and sampling for ADB is best done early in the morning or late afternoon. ADB populations can be assessed visually as well as with a beatsheet (at the time of this study only visual sampling was used). Assessments should be conducted in the top half of cotton plants.

Overall visual sampling was the most efficient sampling method for ADB. However, at the seedling stage visual sampling, beat sheet sampling and suction methods were equally effective. As the plants mature, visual sampling was found to be twice as effective when compared to beat sheet sampling. This means that ADB numbers found with beat sheet sampling, from boll set onwards, should be multiplied by two to get a better estimate of the population in the field.

The decision to control ADB depends on both fruit retention and ADB numbers. However, bear in mind that ADB feeding can be very variable and the cotton plant’s ability to compensate for loss of pin squares, by retaining other squares that may otherwise have been shed, is usually quite good.

Control options

If the decision to control is warranted, it is essential to consider the risk of flaring secondary pests. As ADB are also predators, there is a risk that a decision to control them may increase the chance of mite population build-up. As an occasional pest there are few products registered for their control. A low rate of Fipronil is the softest registered option. The only other registered options, OPs (organophosphates), are not available in the IRMS until late in the season and would be highly disruptive to beneficial insects in the crop.

Article by Moazzem Khan and Susan Maas