A better understanding of the interaction between Bt biopesticides and insecticides is necessary to scrutinize their impact on pests, nontarget organisms, and natural enemy populations. In the present study, these impacts were described using sublethal measurements of Bt biopesticides and combinations of insecticides on the survival of C. includens larvae. The results underscore the need to reconsider the notion that “product efficacy is only directly related to pest mortality.”
Larval survival rates for C. includens of 85–98% on artificial and natural diets (cotton, sunflower, and soybeans) is reported in the literature (Andrade et al. 2016). Those rates are similar to the values obtained in this study. Furtheremore, Our data are consistent with the duration of the larval stage (between 14 and 23.3 days) as reported by Shour and Sparks (1981). The same was observed to larvae from the control; all of them became pupae, according to the data obtained to this species (Barrionuevo and San Blas 2016).
The intrinsic characteristics of each product must be consider to provide valuable insights into our results. The products belong to different formulation types, namely suspension concentrate (SC), wettable powder (WP), emulsifiable concentrate (EC), water-dispersible granules (WG), and solution concentrate (SL). Most treatments belong to the first one (Table 1). For most treatments containing Larvin WG 800® (Lr), larval survival was approximately 69% in the first evaluation. In comparison, for the other product with a neurotoxic mode of action (Belt SC®) (Be), surviving larvae (28% on average) were reported in 10 mixtures (Table 2). Furthermore, the mixture of Th + Av with Th + Lr increased C. includes larval stage until the last and previous evaluations, respectively (Table 3). However, the addition of Larvin WG 800® (Lr) to the first mixture (Th + Lr + Av) reduced the larval stage, as well as when added to the other two mixtures (Di + Lr + Cu and Th + Lr + Pr) (Tables 2 and 3). This effect is likely due to mixing with other products, as it has been reported that the active ingredient of Larvin WG 800® (Thiodicarb) does not affect the duration of the immature stage in Helicoverpa armigera (Hubner, 1805) (Lepidoptera: Noctuidae) (Saber et al. 2013). Another possibility is that, when mixed with other products, formulation (WG) provides a maximum increase in the number of surviving larvae, given the poor dissolution of granules of different sizes (Knowles 2006) or due to dry compaction during manufacturing (Knowles 2008).
Furthermore, most of the insecticides evaluated in the present study (Table 1) targeted the nervous system (Pr, Lr, Ex, and Av). Others targeted the nervous and muscular systems (Am and Be) or the nervous and hormonal systems (Cu) (IRAC, 2020). Changes in nervous and muscular systems, consumption, oviposition, and motor activity as a function of the sublethal effects of the main insecticide were also reported (Karise and Mänd 2015). These authors also pointed out that changes in the nervous and muscular systems affect thermoregulation, breathing, and water balance in insects. Finally, the authors concluded that the most common sublethal effects are changes in the muscular system that cause a collapse in other systems. For example, there could be a loss of spiracle opening and closing control, which increases water losses and may cause the death of insects.
The insect, in turn, can defend itself from infection by the pathogen (Pinos et al. 2021). The immune system is divided into humoral and cellular components, mediated by microbial peptides and hemocytes. The most common types of hemocytes are prohemocytes, granulocytes, plasmatocytes, spherulocytes, and oenocytoids (Strand et al. 2008) Granulocytes and the humoral and cellular systems are responsible for the defense of H. armigera against Bt infection (Wang et al. 2010).
In the first evaluation, the higher average weight of larvae in some treatments may be due to the insect immune system response to Bt infection increasing the food intake as reported by de Oliveira et al. (2013). In addition, products with an ingestion action can contain feed stimulants (Nielsen-LeRoux et al. 2012), which could also increase consumption and, consequently, increase larval weights. Therefore, at adequate quantity and quality, the food stimulants added to the product tank mixtures can increase the consumption of insects.
After ingesting food with Bt, insects can recover from bacterial infection by increasing cell production in the midgut (Castagnola and Jurat-Fuentes 2016). The presence of granules (spherites) covering the epithelial cells of the middle intestinal tract of diamondbacl moth after the ingestion of Dipel WP® was reported (Ribeiro et al. 2013).
An increase in larval stage due to a lower conversion efficiency of ingested food or starvation caused by interruption or decreased feeding was observed by Hanning et al. (2009). The latter assayed chlorantraniliprole, which is the active ingredient in the Prêmio SC®. The same phenomenon was observed for the Foray 48B biopesticide when used against spruce budworm (Moreau and Bauce, 2003).
The active ingredient in Avatar CE® (Av), indoxacarb, has been reported to cause a 2.8- to 3-day increase in the larval stage of Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) (Gamil et al. 2011). In our study, it increased by 12 days (63% longer than the control) for about 20% of the larvae evaluated, but less than 50% (4 specimens) reached the pupal stage (Table 2).
Co-formulants, the so-called inerts, in formulations may contribute to the sublethal effects of active compounds of pesticides. Karise et al. (2015) reported that kaolin increases water loss with consequent longevity shortening in bumble bees. Similar effects were described while studying the impact of diatomaceous earth on the stored grain mite (Cook et al 2008). Detailed information on the co-formulants content of the product would provide valuable information on biopesticides and pesticide combinations.
Finally, 1,192 (62%) of the surviving insects died during the evaluation period of the larval stage, which would increase mortality by 13.5%. Consequently, the sublethal effects would improve the impacts of treatment on insect populations. Guedes et al. (2016) highlighted that sublethal effects modify the population structure of the target insect and interfere with its ecological interactions. Besides, the authors pointed out the common sense that efficacy is intrinsically related to the lethal effects (death), restricts a more holistic and detailed approach to their sublethal effects to a few studies.