16.6. Host-plant resistance to insects
Plant resistance to insects consists of inherited genetic qualities that result in a plant being less damaged than another (susceptible one) that is subject to the same conditions but lacks these qualities. Plant resistance is a relative concept, as spatial and temporal variations in the environment influence its expression and/or effectiveness. Generally, the production of plants resistant to particular insect pests is accomplished by selective breeding for resistance traits. The three functional categories of plant resistance to insects are:
- antibiosis, in which the plant is consumed and adversely affects the biology of the phytophagous insect;
- antixenosis, in which the plant is a poor host, deterring any insect feeding;
- tolerance, in which the plant is able to withstand or recover from insect damage.
Antibiotic effects on insects range from mild to lethal, and antibiotic factors include toxins, growth inhibitors, reduced levels of nutrients, sticky exudates from glandular trichomes (hairs), and high concentrations of indigestible plant components such as silica and lignin. Antixenosis factors include plant chemical repellents and deterrents, pubescence (a covering of simple or glandular trichomes), surface waxes, and foliage thickness or toughness — all of which may deter insect colonization. Tolerance involves only plant features and not insect—plant interactions, as it depends only on a plant’s ability to outgrow or recover from defoliation or other damage caused by insect feeding. These categories of resistance are not necessarily discrete — any combination may occur in one plant. Furthermore, selection for resistance to one type of insect may render a plant susceptible to another or to a disease.
Selecting and breeding for host-plant resistance can be an extremely effective means of controlling pest insects. The grafting of susceptible Vitis vinifera cultivars onto naturally resistant American vine rootstocks confers substantial resistance to grape phylloxera (Box 11.2). At the International Rice Research Institute (IRRI), numerous rice cultivars have been developed with resistance to all of the major insect pests of rice in southern and south-east Asia. Some cotton cultivars are tolerant of the feeding damage of certain insects, whereas other cultivars have been developed for their chemicals (such as gossypol) that inhibit insect growth.
In general, there are more cultivars of insect-resistant cereal and grain crops than insect-resistant vegetable or fruit crops. The former often have a higher value per hectare and the latter have a low consumer tolerance of any damage but, perhaps more importantly, resistance factors can be deleterious to food quality.
Conventional methods of obtaining host-plant resistance to pests are not always successful. Despite more than 50 years of intermittent effort, no commercially suitable potato varieties resistant to the Colorado potato beetle (Chrysomelidae: Leptinotarsa decemlineata) have been developed. Attempts to produce potatoes with high levels of toxic glycoalkaloids mostly have stopped, partly because potato plants with high foliage levels of glycoalkaloids often have tubers rich in these toxins, resulting in risks to human health. Breeding potato plants with glandular trichomes also may have limited utility, because of the ability of the beetle to adapt to different hosts. The most promising resistance mechanism for control of the Colorado potato beetle on potato is the production of genetically modified potato plants that express a foreign gene for a bacterial toxin that kills many insect larvae (Box 16.5). Attempts to produce resistance in other vegetables often have failed because the resistance factor is incompatible with product quality, resulting in poor taste or toxicity introduced with the resistance.
