To help understand the underlying basis of the resistance to pirimicarb, and hence improve the chance of effective management, Dr Robin Gunning and Dr Graham Moores (Rothamstead, UK) did a preliminary screen on resistant clones. They found that the underlying resistance biochemistry (resistance mechanism(s)) is probably a target-site insensitivity and furthermore suggested that pirimicarb resistance can be maintained without selection. Once selected resistance should therefore persist through the season and overwinter causing ongoing and progressively worsening control problems.

Cotton Aphid: Due to its ability to rapidly develop resistance cotton aphid (Aphis gossypii Glover) is world-wide the major aphid pest of cotton. It causes significant problems in Thailand, The Sudan, Russia and the USA. In Australian cotton it is a persistent secondary pest with potential to become a major pest due to resistance.

Resistance allows the uncontrolled increase in aphid numbers causing their sugary honeydew secretions to contaminate the cotton lint. This causes significant discounts due to the need to clean the cotton before it can be spun in today’s high speed spinning equipment. Such a scenario happened recently in the USA causing an immediate and substantial downgrading of the contaminated lint value. Recent Australian research has also confirmed that uncontrolled aphid outbreaks earlier in the cotton season can significantly impede plant development, through aphids competing for plant assimilate causing dramatic yield reductions (up to 75 %). As cotton production in Australia is a 1.5 billion-dollar industry uncontrolled resistance would be a national disaster, due to loss of yield but more significantly damage to our international reputation as a producer of ‘clean’ cotton.

Resistance management in Australia is further complicated because cotton aphid reproduces asexually with female aphids giving birth to live female ‘clones’. Unmanaged insecticide use can rapidly kill susceptible clones, leaving only resistant clones. Further use of the same insecticide group then may select for genotypes within the resistant clones that have small transcription errors or mutations that favour fitness or confer other mechanisms of resistance. Selection of resistant clones allows the rapid proliferation of resistance because there is no outcrossing with susceptible wild types. A second pest aphid, the green peach aphid is also a sporadic but at times damaging pest in cotton. This species is already widely resistant to a range of insecticides and can present major control problems for cotton growers.

Two-spotted mite: Worldwide over 33 species of mites attack cotton, but in Australia damage is due mainly to three species of Tetranychid mite, the banana spider mite (Tetranychus lambi), the bean spider mite (T. ludeni) and the two-spotted mite (T. urticae), though the latter is by far the most common. Two-spotted mite is extraordinarily adaptable and renowned for developing resistance to chemicals used for its control. Two-spotted mite is acknowledged as the most serious world mite pest of agriculture due to its ability to develop resistance and in Australia uncontrolled resistance has rendered some horticultural crops uneconomic. Two-spotted mite has been the dominant mite pest in cotton since the early 1980s. Uncontrolled populations can cause significant losses of yield and fibre quality.