The effects of adjuvants on Steinernema carpocapsae efficacy against Chrysodeixis includens and suspension stability

Authors: Zhang, Minling; Spaulding, Nathan; Reddy, Gadi; Shapiro Ilan, David

Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2025
Publication Date: 1/30/2026
Citation: Zhang, M., Spaulding, N.R., Reddy, G.V., Shapiro Ilan, D.I. 2026. The effects of adjuvants on Steinernema carpocapsae efficacy against Chrysodeixis includens and suspension stability. Journal of Nematology. 57(1):1-12. https://doi.org/10.2478/jofnem-2025-0063.
DOI: https://doi.org/10.2478/jofnem-2025-0063

Interpretive Summary: The soybean looper is a significant pest of row crops, usually controlled with chemical insecticides. Due to environmental and regulatory concerns, alternative solutions are necessary. Entomopathogenic nematodes (EPNs), commonly referred to as beneficial nematodes, provide a safe and environmentally friendly alternative for pest control. This study evaluated the effects of adjuvant treatments—0.3% xanthan gum combined with 0.3% Tween 80 (Xanthan & Tween) and 0.3% or 1.0% fire retardant gel Barricade (with or without Tween)—on the efficacy of Steinernema carpocapsae against C. includens larvae under laboratory and greenhouse conditions. This study highlights that both adjuvant treatments, Xanthan & Tween, and 0.3% Barricade, can improve the efficacy of EPNs against fifth instars of the soybean looper, compared to EPNs applied without adjuvants under laboratory conditions. However, in greenhouse experiments, only Xanthan & Tween demonstrated a significant enhancement. Further research showed that the addition of 0.3% Tween to 0.3% or 1.0% Barricade can also increase the efficacy of EPNs against fifth instars of the soybean looper, compared to EPNs without adjuvants, Barricade-only treatments, or water controls. Furthermore, laboratory results on EPN sedimentation indicate that adjuvant treatments, including Xanthan & Tween, and Barricade & Tween, can effectively retard sedimentation on EPNs.

Technical Abstract: Chrysodeixis includens, a significant pest in southern U.S. soybean production, has developed resistance to many conventional insecticides, prompting exploration of alternative control strategies such as entomopathogenic nematodes (EPNs). Aboveground application of EPNs is challenged by the EPN’s susceptibility to UV radiation and desiccation. Therefore, exploration of protective adjuvants are warranted. This study evaluated the effects of adjuvant treatments—0.3% xanthan gum combined with 0.3% Tween 80 (Xanthan & Tween) and 0.3% or 1.0% fire retardant gel Barricade (with or without Tween) on the efficacy of Steinernema carpocapsae against C. includens larvae under laboratory and greenhouse conditions. In laboratory trials, the addition of Xanthan & Tween significantly enhanced insect mortality compared to EPNs without adjuvants at 24 and 72 hr post-application after 1 hr of EPN exposure, while 0.3% Barricade showed no effect. Extended exposure durations of 4 and 8 hr with both Xanthan & Tween, and Barricade, significantly increased insect mortality than EPNs without adjuvants at 24 and 72 hr post-application. Further extending exposure to 24 hr significantly increased mortality than EPNs without adjuvants at 24 hr post-application for both treatments, although no significant effects were observed at 72 hr. Additionally, only the EPN with adjuvant treatments caused significantly higher inscect mortality then no-EPN controls at 24 hr post-application for 1, 4 and 8 hr exposure durations. In greenhouse trials, the adjuvant-enhancing effect varied under different conditions. Xanthan & Tween could increase EPN effectiveness compared to EPNs without adjuvants, whereas 0.3% Barricade had no observable effect. The optimization trials demonstrated that additional 0.3% Tween to either 0.3% or 1.0% Barricade significantly enhanced the efficacy of EPNs, compared to EPNs without adjuvants, Barricade-only treatments, or the water controls. Xanthan & Tween at concentrations of 0.1–0.3%, and Barricade & Tween at 1.0–2.0%, effectively reduced sedimentation of S. carpocapsae, resulting in enhanced suspension stability. This study supports the use of adjuvant-enhanced EPN formulations as a promising biological pest management approach in soybean systems challenged by insecticide resistance.