absoluta is to use biological control, but finding an effective natural enemy for a new, invasive pest is not an easy task as a pest often has tens to hundreds of species attacking it 6. One of the alternatives for controlling T.

As the larvae of the pest spend most of their time within the leaf or in the fruit, chemical control is difficult and demands up to five sprays per week and 36 times per production cycle of 12 weeks to be effective and this results in quick development of resistance to pesticides, environmental pollution and human health risks 9. Without control methods, the pest may lead to a complete loss of yield. absoluta was limited to South America until 2006, then the species was accidentally introduced into Spain and currently occurs on most continents and is still spreading at a high rate 7, 8. The need for biological control of Tuta absoluta Although the paper focuses on biological control of pests, the same method can be used, for example, to estimate population reduction effects of invasive organisms on native organisms in natural and agricultural ecosystems. In this paper we discuss a new method for efficacy evaluation of candidate organisms for use in the reduction of populations of pests and apply this method to natural enemies of a serious, quickly spreading invasive pest over the world, the South American tomato moth Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) 7, 8. In biological control, an organism (a natural enemy) is used to reduce the population density of another organism (a pest) 6, which contributes to manage native and exotic pests in natural and managed ecosystems. According to Bellows and Hassell 5 “population equilibria and population regulation lie at the heart of biological control”. In several disciplines of applied ecology, population regulation theories are playing an important role, including nature conservation, restoration of biodiversity, ecology of reintroduction of locally extinct species to their previous habitats, invasive species biology and biological control. For an extensive discussion of the population regulation debate see Turchin 4. However, ecologists today are of the opinion that perfect density dependence does rarely or not occur, and that populations are regulated by interacting density-independent and density-dependent factors. Heated debates took place at congresses and in the literature, which still continue. From the 1930s to the 1990s several theories emerged, with as the two most important ones the density-dependent regulation of populations 2 and population limitation by density-independent factors 3. Since 1798 1, one of the key questions in ecology has been how populations of animals and plants are regulated. Other species showed a high pest reduction capacity and their potential for practical application can now be tested under commercial crop production conditions.

Several natural enemies had pest kill rates that were too low to be able to theoretically reduce the pest population below crop damaging densities. Pest kill rates of six species of predators and seven species of parasitoids of Tuta absoluta were calculated and compared. The pest kill rate is the average daily lifetime killing of the pest by the natural enemy under consideration. Next, we propose to use an aggregate evaluation criterion, the pest kill rate, to compare the pest population reduction capacity of species not eliminated during the first selection.

Evaluation criteria help in a first selection to remove clearly ineffective or risky species from the list of candidates. Finding an effective biological control agent among the tens to hundreds of natural enemies of a pest is a daunting task. Ecologists study how populations are regulated, while scientists studying biological pest control apply population regulation processes to reduce numbers of harmful organisms: an organism (a natural enemy) is used to reduce the population density of another organism (a pest).

Wild 9 control enemies