Figure 1. Comparison between chemical and biological control of the spider mite Tetranychus urticae in a commercial cucumber crop in Finland. D, treatment with the acaricide dicofol. The predatory mite was Phytoseiulus persimilis. Economic threshold level of spider mite is 20 mites per 5 cm². The predatory mite was introduced in the middle of February, and provided a season-long, effective control of the pest. Reproduced with permission from Markkula et al. (1972).

Figure 2. (a) Number of introductions of arthropod agents in classical biological control per decade, as recorded in the BIOCAT database (CAB International, UK). (b) Proportions of introductions in classical biological control for each decade leading to any record of nontarget effect. Adapted from Lynch et al. (2001).

Figure 3. An example of transient effects in classical biocontrol. Relative population densities of the target host (H), parasitoid (P), and a nontarget host (N) over time (generations). As host numbers decline and parasitoid numbers temporarily increase, nontargets are affected and decline to a minimum. After that the parasitoid numbers fall, and the nontarget recovers close to its previous level. Adapted from Lynch and Ives (1999).