Mulching Effect on Lettuce Insect Pests and Natural Enemies: Silver Plastic, Rice Straw and Sawdust Compared in Lalitpur, Nepal

What is the effect of mulching on lettuce insect pests?

Mulch type changed which insects built up on spring lettuce, and in opposite directions for different pest groups. In a 2025 field trial at the Vegetable Crops Development Centre (VCDC), Khumaltar, Lalitpur, Nepal, silver-on-black plastic mulch consistently kept aphids, cutworms and Helicoverpa armigera lowest. Rice straw mulch supported the most ladybird beetles (coccinellid predators) but also the most mealybugs and cutworms. At the final harvest, rice straw (13.07 t ha^-1) and silver plastic (12.84 t ha^-1) gave the highest fresh yields against 7.09 t ha^-1 in bare soil, and sawdust returned the best benefit–cost ratio (1.83).

The study was published in Tropical Agrobiodiversity 7(2): 128–134 (DOI 10.26480/trab.02.2026.128.134). Rabina Pokharel is first author; Bishal Neupane is second and corresponding author, with Dipesh Gyawali and Pawan Sapkota as co-authors. The article's contribution statement credits Pokharel with conceptualisation, investigation, data curation, formal analysis and the original draft, and Neupane with formal analysis, review and editing, and project administration.

Why study mulching for lettuce pest management in Nepal?

Lettuce is eaten raw, so insecticide residues on the leaves are a direct consumer concern, and heavy spraying also harms beneficial insects. The paper notes that Nepalese vegetable growers rely heavily on synthetic insecticides, and that lettuce cultivation has expanded quickly in Nepal, with peri-urban demand in the Kathmandu Valley driving production around Lalitpur. Mulching is already used for moisture conservation, temperature regulation and weed control. Reflective silver films have been reported to deter aphids and whiteflies by disrupting their visual host-finding, while organic mulches can shelter both pests and predators. The authors found no earlier Nepalese study that assessed pests, natural enemies, growth, yield and economic return together under different mulches, which is the gap this trial addresses.

How the experiment was designed

• Site and season: VCDC Khumaltar, Lalitpur (about 1,350 m above sea level), mid-February to mid-May 2025.
• Crop: lettuce cv. Green Wave, transplanted at 25 days after sowing at 30 cm × 30 cm.
• Treatments: T1 black plastic; T2 silver-on-black plastic (silver side up); T3 rice straw (5 t ha^-1); T4 sawdust (4 t ha^-1); T5 unmulched control. Plastic films were 25 microns thick.
• Design: RCBD, five treatments, four replications (20 plots of 1.2 m × 1.2 m, 16 plants each).
• Sampling: five marked plants per plot, visual whole-plant counts between 7 and 9 am at 15, 30 and 45 days after transplanting (DAT).
• No pesticides were applied at any point, so the counts reflect unmanaged pest and predator populations.
• Analysis: ANOVA for RCBD in R with the agricolae and doebioresearch packages; means separated by Duncan's Multiple Range Test (DMRT) at P ≤ 0.05.

Results: insect pests and natural enemies under each mulch

Mulch type significantly affected every pest group at every observation date. The table shows mean counts per plant at 45 DAT, from the article's Table 1. Values sharing a letter in a row do not differ significantly (DMRT).

Source: Pokharel et al. (2026), Table 1. Values are arithmetic means of untransformed counts.

Silver plastic carried no aphids at 15 DAT and no cutworm larvae or H. armigera larvae at 30 DAT. Black plastic, by contrast, had the highest aphid count of any treatment by 45 DAT (3.80 per plant), about 1.6 times the bare-soil control. The authors suggest that black film heats the soil and canopy, which can speed aphid development, but note that this remains provisional because soil and canopy temperature were not measured. The difference between the two plastic films indicates that mulch colour, not just the presence of a physical barrier, shaped the insect community.

Rice straw was the only treatment in which coccinellid numbers were consistently and significantly higher than in the control, pointing to a role in conservation biological control. The strong aphid suppression under silver plastic happened without any increase in ladybirds, which suggests physical deterrence rather than extra predation, although the paper states this was not tested directly.

Lettuce growth, yield and economics

All mulches improved every growth parameter over bare soil. Sawdust led early growth, with plants 18.44 cm tall and 19.25 leaves per plant at 45 DAT. Yield was taken over three harvests at 35, 45 and 55 DAT.

Source: Pokharel et al. (2026), Table 3. Cumulative yield is the sum of three harvests and was not analysed statistically. BCR = gross revenue / total variable cost, at a farm-gate price of NRs 45 per kg.

The authors suggest the weaker final harvest under sawdust, despite its early growth advantage, may relate to nitrogen immobilisation as a high C:N material decomposes, but they flag this as a hypothesis because soil nitrogen was not monitored. The near-unity BCR of the unmulched control (1.02) shows how marginal spring lettuce was without any mulch at this site. The BCR for silver plastic also does not include any saving from reduced insecticide use, which the authors recommend adding to future economic analyses.

What this means for lettuce growers

No single mulch suppressed every pest. The paper's practical recommendation is to choose mulch according to the dominant pest group and local material availability:

• Silver-on-black plastic where aphids and caterpillars such as H. armigera and cutworms drive losses. It gave strong pest suppression, a competitive final yield and the second-highest BCR (1.77).
• Rice straw or sawdust where cost, material availability and conservation of predatory beetles matter most, accepting higher mealybug and cutworm pressure under organic mulch.

What this means for researchers

• Measure soil temperature and moisture directly; microclimate is the proposed mechanism behind nearly every effect, but it was inferred here rather than measured.
• Apply a square-root transformation to count data, or use count models. The authors note that their counts were analysed untransformed, so significance levels for insect variables should be read as indicative.
• Identify insects to species and monitor more natural enemies (parasitoids, lacewings, ground beetles), and separate marketable from total yield.

Pest management questions in another crop are covered in farmers' perception and management of fall armyworm in Chitwan maize. For the statistics side, see analysing RCBD trials in R with agricolae.

Limitations

The trial covered one season at one site with one cultivar. Soil temperature and moisture were not measured. Insects other than H. armigera were identified only to family, and visual counts estimate relative rather than absolute abundance and under-represent nocturnal and mobile insects. Only coccinellids were monitored as natural enemies, and no formal correlation between pests and predators was computed.

How to cite this study

Pokharel, R., Neupane, B., Gyawali, D. and Sapkota, P. (2026). Effect of Mulching Materials on Population Dynamics of Insect Pests and Natural Enemies and on Growth and Yield of Lettuce (Lactuca sativa L.) in Lalitpur, Nepal. Tropical Agrobiodiversity, 7(2): 128–134. https://doi.org/10.26480/trab.02.2026.128.134

All of Bishal Neupane's articles are on the publications page, with field and analysis methods on the methods page. Supervisors and collaborators are welcome to make contact through the contact page.