Optimizing Avermectin Nano‑Delivery: Impact of Particle Size on Release, Stability, and Pest Control

Background

Pesticides are pivotal for plant disease and pest management, yet conventional formulations suffer from poor dispersion, rapid degradation, and high loss rates (up to 70–90 %) due to biodegradation, photolysis, evaporation, runoff, and groundwater leaching. These challenges compromise food safety and environmental quality, underscoring the need for advanced delivery strategies.

Nanotechnology offers a promising avenue to enhance pesticide performance. Nanoparticle formulations increase surface area and improve spatial distribution on leaf surfaces, while polymeric nano‑carriers enable slow, controlled release of active ingredients. Polylactic acid (PLA), a biodegradable, FDA‑approved polymer, has proven effective as a carrier for bioactive molecules and is increasingly explored for pesticide encapsulation because of its environmental friendliness, low cost, and scalability.

Nanoparticle delivery expands contact area between pest and pesticide, boosting adhesion and penetration. Avermectin (Av) is a broad‑spectrum, low‑toxicity biopesticide, yet it degrades rapidly under UV and has a short aqueous half‑life. Microencapsulation has been attempted, but resulting capsules (1–5 µm) are large and uneven, limiting dispersion and efficacy. Limited work exists on Av nano‑delivery systems with controlled size distribution.

This research aims to synthesize Av nano‑delivery systems with varying particle sizes via emulsion polymerization using PLA, and to assess how size influences release behavior, photostability, and biological activity.

Experimental

Materials

PLA and Av were supplied by Nature Works and Qilu Pharmaceutical Co., Ltd. (Inner Mongolia, China). Polyvinyl alcohol (PVA, 87–90 % hydrolyzed, M_w 30 000–70 000) was purchased from Sigma‑Aldrich Shanghai. Gelatin was obtained from Sinopharm Chemical Reagent Co., Ltd. Dialysis membranes came from Beijing Tianan Technology Co., Ltd. All other reagents were analytical grade, and Milli‑Q water (18.2 MΩ cm) was used throughout.

Preparation of Avermectin Nano‑delivery System

An oil‑in‑water (O/W) emulsion was formed by combining PLA and Av dissolved in methylene chloride (oil phase) with a water phase containing gelatin and PVA. The oil phase was dripped into the water phase under high‑shear emulsification (FA25, FLUKO) to produce a coarse emulsion, then homogenized by ultrasonication (JY 92‑IIN). The resulting emulsion was stirred overnight to evaporate the solvent, collected by centrifugation, washed three times with deionized water, and freeze‑dried to a free‑flowing powder, stored at 4 °C.

Characterization of Nano‑delivery Systems

Particle morphology was examined by scanning electron microscopy (SEM, JSM‑6700 F) after platinum sputter coating. Size distributions were measured by dynamic light scattering (Zetasizer NanoZS90) at 25 °C.

Determination of Avermectin Loading

Av content was quantified by UV–vis spectroscopy (245 nm). Samples were weighed, dissolved in chloroform, evaporated, re‑dissolved in methanol, filtered, and analyzed.

Controlled Release Studies

Samples were suspended in 10 mL of 1:1 ethanol/water, placed in a dialysis bag, and submerged in 90 mL of the same medium. At set intervals, 5 mL aliquots were replaced with fresh solvent, and Av concentration was measured by UV–vis. Technical abamectin (TC) served as control.

Photolysis Behavior

Av photolysis was assessed by irradiating methanol/water solutions under a 500 W UV lamp (λ = 365 nm) for up to 72 h, sampling every 12 h.

Stability Tests

Samples were stored at 0 ± 2 °C for 7 days and 54 ± 2 °C for 14 days, then Av content was re‑measured.

Bioassays

Leaf‑dip bioassays against cabbage aphid larvae were performed. Leaves were immersed in Av suspensions containing Triton X‑100, dried, and exposed to 48 h incubation (25 °C, 75 % RH). Mortality was recorded, and LC₅₀ values were calculated with DPS v12.01. Commercial WDG was used as control.

Results and Discussion

Construction and Characterization of the Avermectin Nano‑delivery System

Figure 1 outlines the synthesis workflow. By varying the PVA/gelatin ratio, particle sizes from 344 to 827 nm were achieved (Figure 2). Av loading ranged from 33.4 % to 57.5 % (Figure 3). All particles were smooth, spherical, and uniformly dispersed.

Schematic of the Av nano‑delivery system preparation.

SEM images (a–d) and size distributions (e) of Av nano‑delivery systems.

Av loading across particle sizes.

Avermectin Release from the Nano‑delivery System In Vitro

Release profiles (Figure 4) reveal that technical Av releases almost entirely within 25 h, whereas nano‑systems exhibit a biphasic release: an initial burst followed by sustained release over 240 h. Smaller particles (344 nm) released 79.4 % of Av, compared to 53.2 % for 827 nm particles, indicating that higher surface area accelerates diffusion. Thus, particle size is a key lever for tailoring release kinetics.

Release behaviors of Av nano‑delivery systems in ethanol/water (1:1, v/v) over 200 h.

Biological Activity

Bioassays against aphids (Figure 5) show a clear trend: LC₅₀ values decrease as particle size shrinks. Smaller particles enhance dispersion, wettability, and retention, thereby increasing bioavailability. All nano‑systems outperform commercial Av WDG, confirming that nano‑encapsulation boosts efficacy.

Bioassay results of Av nano‑delivery systems with varying particle sizes.

UV‑Shielding Properties of Avermectin in the Nano‑delivery System

Photolysis experiments (Figure 6) demonstrate that the nano‑delivery system protects Av from UV degradation: after 48 h, only 18.7 % photolysis occurs versus 46.7 % for commercial WDG; at 72 h, 25.6 % versus 51.5 %. The polymer wall effectively shields the active ingredient.

Comparison of Av photolysis percentage with commercial WDG and nano‑delivery system under UV irradiation.

Storage Stability

Stability tests (Figure 7) indicate that Av loading remains stable at 0 °C and 25 °C. A modest loss (< 5 %) occurs after 14 days at 54 °C, attributable to thermal degradation. Overall, the nano‑delivery system exhibits excellent storage stability.

Stability of Av nano‑delivery system at different storage temperatures.

Conclusions

We successfully fabricated avermectin nano‑delivery systems with controlled particle sizes using emulsion polymerization and PLA as a biodegradable carrier. Particle size dictates release rate, UV protection, and biological activity: smaller particles release Av more rapidly, enhance pest control, and offer superior photostability. These nano‑systems overcome limitations of conventional biopesticides—such as rapid degradation, soil adsorption, and short action duration—thereby improving efficacy, reducing application frequency, and minimizing environmental impact.