Tolerance: the forgotten child of plant resistance

Abstract

Plant resistance against insect herbivory has greatly focused on antibiosis, whereby the plant has a deleterious effect on the herbivore, and antixenosis, whereby the plant is able to direct the herbivore away from it. Although these two types of resistance may reduce injury and yield loss, they can produce selection pressures on insect herbivores that lead to pest resistance. Tolerance, on the other hand, is a more sustainable pest management strategy because it involves only a plant response and therefore does not cause evolution of resistance in target pest populations. Despite its attractive attributes, tolerance has been poorly studied and understood. In this critical, interpretive review, we discuss tolerance to insect herbivory and the biological and socioeconomic factors that have limited its use in plant resistance and integrated pest management. First, tolerance is difficult to identify, and the mechanisms conferring it are poorly understood. Second, the genetics of tolerance are mostly unknown. Third, several obstacles hinder the establishment of high-throughput phenotyping methods for large-scale screening of tolerance. Fourth, tolerance has received little attention from entomologists because, for most, their primary interest, research training, and funding opportunities are in mechanisms which affect pest biology, not plant biology. Fifth, the efforts of plant resistance are directed at controlling pest populations rather than managing plant stress. We conclude this paper by discussing future research and development activities.

Keywords: Antibiosis; Antixenosis; Insect resistance; Integrated pest management; Plant breeding.

Figure 1. The damage curve relating intensity of injury to yield.

Figure 2. The damage curve showing different portions where tolerance can be expressed.

(A) shows extending the initial zero slope of the damage curve, i.e., no damage per unit injury may be expressed at higher levels of injury for tolerant plants than for nontolerant plants; (B) shows that because this area is curvilinear (with a negative decreasing slope), tolerant plants may express less damage per unit injury; (C) shows that the curvilinear portion may be extended into higher levels of injury; (D) shows that the constant, negative slope (constant damage per unit injury) may have a less negative slope for tolerant plants; (E) shows that the linear portion may be shorter; (D) shows that desensitization and inherent impunity may occur at a higher yield; (F) shows that overcompensation (increasing yield per unit injury), may be expressed by both tolerant plants and nontolerant plants, but tolerant plants may express a higher yield increase per unit injury.

Figure 3. The relationship between injury (often expressed as number of insects), time, and the economic injury level with and without tolerance.