Pesticides are widely used in farms and quickly degraded after application; hence, target and non-target insects are often exposed to low (and largely sublethal) residues 24–26. The importance of such exposure is prevalent in the field and should not be neglected as it may affect the management of pest species 26,27. The intent of the present work was to evaluate the effect of such low insecticide concentrations on a pest species, A. gossypii, and one of its important natural enemies, the parasitoid wasp Aphidius colemani. The results obtained indicated that both organisms are significantly compromised by the insecticide mixture, following a concentration-dependent trend.
Previous studies have demonstrated that, if the population growth of cotton sucking pests is controlled, natural enemies will be able to control the population of these species 28. However, our results dispute such a notion and are consistent with those of Majidpour, et al. 29, who reported that a sublethal concentration (LC10 and LC30) of thiacloprid + deltamethrin significantly reduces the life table parameters of A. gossypii and Aphidius flaviventris (Hymenoptera: Aphelinidae). Furthermore, Shi, et al. 30 reported that fecundity and longevity of A. gossypii at LC20 were significantly reduced by thiacloprid. Our results are also in line with Vojoudi and Saber 31, who reported that a sublethal concentration of thiacloprid significantly increased pupal and larval development times and reduced adult longevity and fecundity of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Similarly, Miao, et al. 32 observed that sublethal concentrations of thiacloprid on Sitobion avenae (Fabricius) (Hemiptera: Aphididae) reduced r, R0 and increased generation time.
The significant effects of low concentrations of insecticide mixture affect not only the cotton aphid, but also other species. Their negative effects on the aphid parasitoid and biocontrol agent Aphidius colemani were also significant, especially on the aphid LC30. The time-mortality results also indicated that LC30 was highly lethal to parasitoid adults up to 24 hours after the application. Then, lethality dropped drastically. These findings are also consistent with those reported for other species. It has been reported that the pyrethroid cyhalothrin may reduce the population of natural enemies, which may favor the growth of A. gossypii population 12. Mardani, et al. 33 found that thiacloprid + deltamethrin presented greater negative impact than pirimicarb and pymetrozine on Lysiphlebus fabarum (Marshall) (Hymenoptera: Aphididae) and reduced its survival, adult emergence and fecundity.
Thiacloprid + deltamethrin also compromised the population parameters of L. fabarum, which is consistent with our findings for A. colemani. In another study, thiacloprid was tested on Trichogramma evanescens (Westwood) (Hymenoptera: Trichogrammatidae) and reduced adult emergence and parasitism by this species, although other species of this genus may not be as strongly affected. Mead-Briggs 34 found that the adult stages were significantly affected by thiacloprid, compared to the pre-imaginal stages in Aphidius rhopalosiphi (DeStefani-Perez) (Hymenoptera: Braconidae). These findings suggest that Aphidius colemani is more susceptible to the insecticide mixture thiacloprid + deltamethrin than other parasitoids.
Thiacloprid + deltamethrin presented high potential to control A. gossypii, even at sublethal concentrations. Given the results, thiacloprid + deltamethrin proved to be an effective insecticide for controlling A. gossypii. Nonetheless, the negative impact of insecticide mixture on life table parameters of Aphidius colemani is a concern. Thiacloprid + deltamethrin consists of a neonicotinoid and a pyrethroid, each with a different mode of action. Neonicotinoid insecticides affect acetylcholine receptors, while pyrethroids are sodium channel modulators, and both affect the insect nervous system. While affecting the nervous system, these compounds are also able to affect other physiological systems via indirect action, or via secondary action. Thus, the decreased fecundity and longevity in adult insects may result from failure in the neurosecretory system under the influence of these insecticides 31,35,36. Regardless, the findings of the present study indicate that thiacloprid + deltamethrin is effective against A. gossypii, but the combined use of this pesticide with Aphidius colemani is not suitable for pest management programs.