The results of the present study showed that exposure to essential oils and insecticides differed in the type of functional response. Studies have indicated that type II and III responses are more frequent among parasitoids (Paes et al. 2018; Rostami et al. 2018; Ranjbar et al. 2018, Saber et al. 2020).
In the type I response, parasitism increases linearly with the increase in their density. The identified type II functional response suggests that T. pretiosum parasitoids increased their parasitism rate when host density increased, until they reached maximum reproductive capacity (Fernández-Arhex and Corley 2003; Jarrahi and Safavi 2016; Paes et al. 2018) therefore, there is a physiological reduction in the search rate over time as the eggs are successively depleted (Mills and Lacon, 2004).
T. brassicae (Hymenoptera: Trichogrammatidae) exposed to insecticides, fipronil and diazinon revealed a type II functional response in the control and fipronil, and type III to diazinon (Saber et al. 2020). Two botanical insecticides, Dayabon® and Palizin®, were evaluated on the functional response of Lysiphlebus fabarum (Hymenoptera: Braconidae) and the results revealed a type II functional response in all experiments, including the control (Jam 2018); these same botanical insecticides were also evaluated for the parasitoid Habrobracon hebetor (Hymenoptera: Braconidae), where Palizin® showed a type II response and Dayabon® type III (Asadi et al. 2020).
It is assumed that parasitoids that present a type III functional response have better opportunities to regulate their host than type II parasitoids (Ghorbani et al. 2019), however, the functional response is one of the characteristics considered related to parasitoid success, only the shape of the curves of the types of functional response cannot be attributed to the performance of the parasitoids in biological control, other aspects such as, time and rate of release, quality and quantity of parasitoids, climatic conditions must be considered (Rostami et al. 2018 ) .
The functional response relies on two important aspects, including search efficiency (a) and manipulation time (Th). The rate of these parameters and the type of functional response in predators and parasitoids are influenced by different factors (Farrokhi et al. 2010; Milanez et al. 2018). One is the sublethal concentrations of chemicals used to control insect pests. In this study, O. majorana and C. officinallis showed sublethal effects on T. pretiosum functional response parameters. Wasps exposed to the oils had longer and shorter manipulation times for O. majorana and C. officinallis, respectively, compared to the control. In the insecticides azadirachtin and deltamethrin, there was an increase in handling time and the attack rate was lower for both insecticides used, when compared to the control.
The reduction in handling time causes an increase in oviposition rates due to exposure to oils, compared to the control. Handling time includes time to find the host, oviposition, rest, and time to ingest water or sap. On the other hand, an increase in the handling time of T. pretiosum in the control and insecticides may be due to the prolongation of each of the items mentioned, and consequently, low rates of parasitism. (Sule et al. 2014; Jam and Saber 2018). According to Jarrahi and Safavi (2016), azadirachtin had no significant adverse effect on functional response parameters (“a” and “Th”) in the parasitoid H. hebetor, while deltamethrin and fenvalerate significantly decreased the handling time of H. hebetor.
The attack rate, which represents the host's search efficiency, in azadirachtin was the lowest in all treatments, indicating that this important parameter was reduced by the tested insecticide, which is a negative aspect and may be enough to compromise the parasitism potential, since this parameter considers the proportion of successful attacks during the search (Oliveira et al. 2006).
The shortest handling time and high search efficiency recorded for C. officinallis can be considered a good result. Furthermore, short handling times and high search efficiency lead to optimal foraging of parasitoids and population stability between parasitoids and hosts (Paes et al. 2018).
The functional response of parasitoids to host population density is also based on reaction to host kairomones (Reznik and Umarova 1991). In the natural environment, insects are attracted by odors, which are usually mixtures of chemicals. In these mixtures, there are compounds that are considered key components and can sometimes cause attraction (Zhu et al. 2016). As the number of eggs increases, the concentration of kairomones increases and, consequently, the percentage of females that parasitize (Reznik and Umarova 1991).
In this study, handling the eggs to separate them according to each density may have influenced the low parasitism, the presence of host body scales left on the eggs, emit volatile compounds, which influence the attractiveness and behavior of the parasitoid. The volatile compounds of substances accumulated on or close to the eggs during oviposition or to fix them to the substrate can act in the short-distance attraction (Carneiro et al. 2006).
The oils tested in sublethal concentrations may have triggered an olfactory response, helping the parasitoids in the process of locating the host, reducing handling time, thus differentiating them from other treatments.
Some plant species (e.g. tomatoes) are known to naturally release specific compounds such as sesquiterpenes, which significantly influence the attraction of Trichogramma species, which uses chemical cues as a search strategy to increase parasitism in eggs (Rani et al. 2017; Gontijo et al. 2019; Souza et al. 2021). C. officinalis and O. majorana oils are mainly composed of sesquiterpenes and monoterpenes, respectively (Arruda et al. 2019; Santana et al. 2022). It is possible that these chemical products combined with the egg could help localize the parasitoid to the eggs, however, such a statement would require further studies.
Additionally, in the numerical response, a population with a large availability of hosts will have a greater chance of surviving and successfully reproducing, which will lead to a population increase (Solomon 1949). Among some aspects, the increase in the rate of parasitized eggs in the treatment with the oils, as a response to the greater supply of hosts, suggests that the parasitoid has the potential for population regulation of the pest (Godfray 1994). As for exposure to insecticides, it is suggested that this parasitoid could not regulate the pest at high densities; being a feature of the type of functional response found for azadirachtin (Type I) where the rate of parasitism is kept constant regardless of host density (Chen et al. 2008).