The demand for vegetables is increasing as consumers seek to diversify their diets for essential nutrients that form healthy diets (Da Wafi et al., 2023; Dubey et al., 2020). Increased vegetable production also mitigates food insecurity and enhances food and economic self-sufficiency. It also generates income, particularly for youth and women throughout the vegetable production chain in rural, urban, and peri-urban areas (Bokelmann et al., 2022; Schreinemachers et al., 2018). In Kenya, the main vegetable crops are tomato (Solanum lycopersicum), cabbage (Brassica oleracea), kale (Brassica oleracea var. acephala), African nightshade (Solanum scabrum), amaranth (Amaranthus spp.), French beans (Phaseolus vulgaris), carrot (Daucus carota), and onions (Allium cepa) (Anyega et al., 2021; Lagerkvist et al., 2012; van der Lans et al., 2012; Ngigi et al., 2011). Kale is mainly grown for household consumption and plays an important role in improving the nutritional balance of diets while also serving as an important source of income for peri-urban farming households (Lagerkvist et al., 2012; Ngigi et al., 2011). The production of vegetables is however constrained by pest infestations and poor soil (Mantovani et al., 2017; Mutiga et al., 2010; Ogol and Makatiani, 2007; Tully et al., 2015). Other challenges include increasingly hot and dry weather conditions that lead to low production (Anyega et al., 2021; Nordey et al., 2017).

Insect pests and diseases have long been recognized as important constraints to vegetable production worldwide, and have been subject to extensive research. However, nematodes, are frequently overlooked, as their symptoms are often mistaken for nutrient deficiencies (Coyne et al., 2018a). This is further compounded by the limited awareness of nematode-related problems among smallholder farmers in sub-Saharan Africa (Coyne et al., 2018a). While plant-parasitic nematode (PPNs) are well known for their detrimental effects on vegetable crops (Sandrine et al., 2023), free-living nematodes (FLNs) are valued for their beneficial roles in the rhizosphere and their positive contribution to soil health and crop productivity (Atandi et al., 2022). FLNs are assigned to different trophic groups, namely, bacterivores, fungivores, predators and omnivores. The contribution of FLNs to the acceleration of nutrient mineralization as well as promoting release of the plant-available nitrogen in soils has been reported in previous studies (Kekelis et al., 2022; Lu et al., 2020; Natalio et al., 2024). For this reason, FLNs community composition is increasingly being used as indicator of soil health, with high densities of these organisms often reflecting improved organic matter content and general soil fertility. Predatory and omnivorous nematodes contribute to biological control by regulating the populations of PPNs (Atandi et al., 2022; Khan and Kim, 2007).

Plant-parasitic nematodes attack underground plant parts, causing crop damage and yield losses, and inducing symptoms that are often mistaken for those caused by abiotic factors, which makes their management challenging (Sandrine et al., 2023). Aboveground symptoms of nematode root damage include nutrient inadequacy, wilting, stunting, poor production, and, in extreme cases, plant mortality (Coyne et al., 2018b). Farmers have relied on synthetic nematicides for PPNs management (Kiriga et al., 2018; Stirling and Pattison, 2008), but these are costly and pose adverse effects to human health, the environment and the microbial biodiversity (Agboyi et al., 2016; Bamboriya et al., 2022; De Bon et al., 2009; Nordey et al., 2017). There is therefore a need to develop sustainable and eco-friendly alternatives to address PPNs management practices. Studies of the efficacy of different cropping systems and organic amendments against PPNs have shown promise from both an economic and ecological perspective (Anedo et al., 2025; Atandi et al., 2022; Dawabah et al., 2019; Paudel et al., 2020).

Vegetable integrated push-pull technology (VIPP) has been reported as a promising strategy and sustainable path to pest management, improved crop productivity and nutritional quality (Chidawanyika et al., 2023, 2025). The VIPP is a form of intercropping of vegetable and cereal crops with pest-attractant and repellent fodder plants. The main crops in these systems are intercropped with leguminous companion plants of the genus Desmodium [Fabaceae] to control the pests of the main crop and suppress parasitic weed growth (Chidawanyika et al., 2023; Khan et al., 2001). The push-pull field is then sown with napier grass (Pennisetum purpureum) or Brachiaria spp. as a border crop, surrounding the whole field (Midega and Khan, 2003). The Desmodium plants produce volatiles that repel various pests while the surrounding grasses serve as alternative attractive ovipositional hosts that are not suitable for full development of larvae (Khan et al., 1997). Apart from pest regulation, the benefits of this system are enhanced safe food production, improved soil fertility and improved overall soil health (Drinkwater et al., 2021; Ndayisaba et al., 2021). This cropping system is gaining increasing interest in agricultural production as it employs on-farm crop diversification to manage pests by leveraging chemically and physically mediated interactions with non-host plants, coupled with the presence of natural enemies (Lang et al., 2022). Although the beneficial effects of VIPP against above-ground pests in East Africa has been demonstrated (Chidawanyika et al., 2025; Khan et al., 2001, 2018; Midega et al., 2018), its effect on belowground pests, particularly nematodes, remains under explored.

The potential of using black soldier fly frass fertilizer (BSFF) as an organic amendment for the control of PPNs was reported by Anedo et al. (2024, 2025). The BSFF, a composted byproduct of black soldier fly larvae, is nutrient-rich and suitable as an organic fertilizer (Beesigamukama et al., 2020a; Oonincx et al., 2015; Tanga et al., 2022). Several studies illustrated the value of frass in improving the growth, yield and nutritional quality of vegetables (Abiya et al., 2022; Agustiyani et al., 2021; Anyega et al., 2021; Borkent and Hodge, 2021; Dzepe et al., 2022; Tan et al., 2021) compared to conventional compost or commercial organic and mineral fertilizers. Similarly, the ability of BSFF to suppress PPNs due to the presence of bioactive compounds such as chitin derived from the pupa exoskeleton was reported (Anedo et al., 2024, 2025; Kisaakye et al., 2024). Chitin acts as a substrate for micro-organisms which parasitize nematodes, thereby reducing PPNs numbers (Kisaakye et al., 2024; Ledchumanakumar et al., 2021). BSFF is also rich in organic matter and nutrients that stimulate the activity of beneficial soil microorganisms, including fungi and bacteria (Beesigamukama et al., 2020b; Yang et al., 2020). Some of these microbes produce nematicidal compounds or infect PPNs, while others are root endophytes, which enhances plant resistance to nematode infection (Kiriga et al., 2018).

Previous studies on the belowground effects of companion crops have largely addressed soil health through fixation of nitrogen and the enhanced accumulation of organic matter and available phosphorus for plants and suppression of Striga weed (Drinkwater et al., 2021; Ndayisaba et al., 2021). There is limited knowledge on the impact of VIPP on the nematodes, particularly community composition, population abundance and diversity as well as pathogenicity. This study was conducted to determine the diversity and population densities of nematodes associated with kale and maize (Zea mays) in VIPP systems augmented with BSFF. Kale crop was used since it is commonly cultivated as leafy vegetable in both regions where the study was conducted. This study presents the first field-based evaluation of a new integration system combining VIPP and BSFF to suppress PPNs in Kenyan vegetable production systems. The objectives were to determine the effects of this integrated system on (i) soil nematode community composition and diversity and (ii) plant-parasitic nematode densities.

Our study represents the first known documentation of the diversity and population densities of nematodes under a vegetable integrated push-pull system combined with BSFF in Kenyan agro-ecosystems. Although 15 genera of PPNs were identified during this study, several including, Globodera, Hoplolaimus, Rotylenchus, Tylenchus, Tylenchorynchus, Paratrichodorus, Xiphinema and Criconema were represented by only a few individuals or low mean densities, suggesting they pose limited or no significant threat to kale or maize crops. Many plant-parasitic genera have previously been reported from vegetable fields elsewhere with some regarded as real threats to the crops (Abiola, 2020; Junior et al., 2021; Ledchumanakumar et al., 2021; Sandrine et al., 2023). This study has demonstrated that PPNs and FLNs are significantly affected by the integration of VIPP and BSFF relative to the baseline populations with the treatments involving the combinations recording the greatest reduction in total PPNs abundance while enhancing the densities of FLNs. Similarly, application of organic amendments decreased the total densities of PPNs and enhanced beneficial nematode numbers in different crop systems (Akhtar and Mahmood, 1996; Rahman et al., 2014). In contrast, control treatments recorded the highest PPNs densities, confirming the susceptibility of monoculture systems to PPNs infestations. Longer periods of pineapple cultivation as a monoculture were associated with increased densities of PPNs and were reported to be detrimental to the beneficial nematodes (Kiriga et al., 2021). In potatoes, Mburu et al. (2020) showed that continued monoculture without practicing fallow or non-host cropping significantly increased the occurrence and densities of potato cyst nematodes consequently leading to severe yield losses.

We detected lower densities of known vectors of plant viruses, Xiphinema (dagger nematodes), Trichodorus and Paratrichodorus (stubby-root nematodes) in VIPP and BSFF-treated plots. This finding suggests the potential of these practices not only to reduce direct nematode damage but also to mitigate indirect crop losses by reducing the risk of virus transmission. While the overall nematode diversity remained relatively high, some PPNs genera (Hoplolaimus, Globodera, Rotylenchus, Paratrichodorus, Xiphinema and Criconema) were consistently observed at low densities. This indicates that the VIPP and BSFF system regulate nematode communities, where damaging species are suppressed but overall biodiversity is enhanced, an important aspect of sustainable soil management (Kiriga et al., 2021). The findings align with prior studies reporting the benefits of organic amendments and diversified cropping systems in reducing nematode damage (Stirling, 2014b). They also highlight the potential of integrating local resources, like BSFF, into smallholder farming systems as a cost-effective and environmentally friendly pest management strategy.

Our study revealed strong positive correlation among several PPNs genera (Pratylenchus, Meloidogyne, Helicotylenchus and Rotylenchulus), indicating that increases in these PPNs genera contributed significantly to overall PPNs population communities (Figure 4). A strong positive correlation was observed between bacterivores and fungivores indicating that these two groups are key contributors to the FLNs community structure. The inverse relationships between PPNs and FLNs in our study suggest a possible antagonistic dynamic, where higher FLNs population may outcompete or predate upon the PPNs. Our findings were similar with several other studies that revealed negative correlation between PPNs and FLNs where organic amendment and cropping systems such as maize-legume intercropping are incorporated (Atandi et al., 2022; Liu et al., 2020; Rosskopf et al., 2020). Omnivores and predators showed weaker or inconsistent correlations with both PPNs and FLNs, implying that their abundance may be influenced by other biotic or abiotic factors. The observed nematode population dynamics in both Embu and Kiambu show contrasting seasonal shifts that likely reflect changes in rhizosphere resource availability, soil structure, and biologically enriched environment. Bacterivores and fungivores remained at 100% occurrence across both sites and seasons, indicating a consistently active microbial food web and steady availability of microbial resources through time (Ferris et al., 2001). Predatory and omnivorous nematodes showed more variation between sites and seasons. In Embu, predatory nematodes occurrence rose markedly from 18% in season 1 to 75% in season 2, while omnivores declined slightly. This pattern suggests a more structured soil food web and greater top-down regulation during the second season, possibly contributing to the reduction of some plant-parasitic genera. In contrast, Kiambu showed an increase in both omnivores (46–75%) and predators (43–54%), pointing to enhanced soil biological balance and a balanced interaction among trophic groups (Ferris et al., 2001; Neher, 2010).

The NMDS ordinations based on Bray–Curtis similarities demonstrated that nematode community composition was strongly influenced by the management practices involved, cropping system practices and organic amendments, in particular with a clear observation that these effects intensified overtime. Similar to our findings, polyculture systems and use of organic amendments have separately been shown to have a long-term negative effect on pest population buildup due to enhanced soil health producing stronger plants that are less susceptible to pest attack as well as enhancing the activity of natural enemies of pest among other mechanisms (Birkhofer et al., 2008; Mutyambai et al., 2019). In both Embu and Kiambu, control and inorganic fertilizer (maize and kale) treatments formed distinct clusters that differed from plots with BSFF and/or companion crops. This pattern indicates that integrated management strategies not only reduced PPNs densities but also shifted the overall soil nematode assemblage toward communities more typical of biologically enriched systems (Cole et al., 2020). Temporal comparisons between season 1 and season 2 revealed that treatment effects on nematode community composition became more pronounced with time. In season 1 at both sites, control plots clustered separately from integrated treatments but still exhibited some overlap, while season 2, displayed clearer segregation of treatments along the NMDS1 axis, with combined VIPP-BSFF systems forming the most distinct clusters. This trajectory implies cumulative or residual effects of organic amendments and diversified cropping systems on soil structure (Alletto et al., 2022; Atandi et al., 2022; Bennett et al., 2012; Zhong et al., 2016).

The findings in the present study demonstrate that the integration of these agro-ecological practices can influence nematode community structure, potentially contributing to more sustainable pest management. Despite the increased nematodes suppression observed in the plots with both the VIPP and BSFF systems, the mechanisms involved have not been properly investigated (Atandi et al., 2022; Qiu et al., 2020). However, previous studies have attributed the cropping systems and organic resources on nematode control to enhanced microbial antagonism through production of antimicrobial compounds, release of nematicidal compounds, lifecycle interruption and disruption of PPNs host-seeking due to diversified host plants by companion crops, habitat modification creating a habitable environment for natural enemies. For example, push-pull cropping system has been shown to enhance beneficial soil microbial and arthropod diversity forming distinct community composition which play an important role in pest and nematodes suppression (Jalloh et al., 2024; Mutyambai et al., 2024; Mwakilili et al., 2021). Similarly, long-term responses to organic inputs have been reported elsewhere, where repeated application of chitin-rich amendments gradually increases populations of beneficial bacterivores and nematode-antagonistic microbes (e.g., Paecilomyces lilacinus, Pochonia chlamydosporia) while suppressing PPNs populations (Zhan et al., 2021).

BSFF is rich in chitin and organic matter. The ability of this fertilizer to reduce nematode numbers has been associated primarily with its high chitin content. Chitin has been linked to increased N levels and low C:N ratios, resulting to nematicidal effects and could lead to enhanced resistance response in plants (Anedo et al., 2024; Rosskopf et al., 2020). Similar to our findings on suppression of PPNs, exposure to chitin-fortified BSFF significantly suppressed the potato cyst nematodes by reducing both number of cysts in soil and their reproduction under field conditions (Anedo et al., 2025) and other nematode species in controlled environments (Anedo et al., 2024; Kisaakye et al., 2024). This may indicate a possible synergistic effect that BSFF organic soil amendments of push-pull cropping systems have. BSFF amendment, likely promotes beneficial soil microbes, including bacteria and fungi that produce nematicidal compounds or act as biological antagonists thereby, leading to competition and predation against PPNs, hence contributing to natural biological control. Previously, chitin has been reported to stimulate populations of nematode-antagonistic microbes, such as Paecilomyces lilacinus and Pochonia chlamydosporia that parasitize nematode eggs and/or juveniles (Alletto et al., 2022; Kisaakye et al., 2024; Suarez-Fernandez et al., 2021; Zhan et al., 2021). Furthermore, the results also aligns with previous reports highlighting the role of organic amendments in enhancing soil suppressiveness towards PPNs (Anedo et al., 2024; Kisaakye et al., 2024; Liu et al., 2020).

Organic amendments and companion cropping can prime plant defense mechanisms, potentially enhancing the production of compounds that increase their resistance against pests. This could be attributed to the activation of plant signaling pathways and release of secondary metabolites or other chemical defenses (Cardoza and Buhler, 2012; Mutyambai et al., 2019). Moreover, various organic amendments contribute to the formation of distinct communities of symbiotic bacteria and fungi. These microorganisms play a significant role in shaping inducible chemical defenses, and the emission of volatile signals by plants (Rowen et al., 2019). Volatile organic compounds derived from Brassicaceae plants were shown to control PPNs through nematicidal activities (Mazzola et al., 2009; Ploeg and Stapleton, 2001; Zasada et al., 2009). Plant extracts have demonstrated the ability to induce synthesis of plant defense proteins, leading to a reduction in Meloidogyne javanica egg hatch and juvenile penetration into tomato roots (Sholevarfard and Moosavi, 2015). Li et al. (2018) indicated that the influence and efficacy of any organic material on a pest is associated with their chemical composition. Compounds with allelopathic properties, were identified in the exudates of Crotalaria juncea, demonstrating nematotoxicity effects (Wang et al., 2001). Likewise, Desmodium root releases an array of secondary metabolites that are known to play a role in shaping the nematodes (Hooper et al., 2015; Tsanuo et al., 2003). These chemicals possess allelopathic and antimicrobial properties and are known to interfere with the chemotaxis, egg hatching and survival of PPNs (Rhodes et al., 2014).

In our study, the densities of FLNs increased under BSFF or companions or the integration. Notably, the bacterivores and fungivores were significantly abundant under BSFF or companions or in the integration and so contributed more to the FLNs composition. This suggested that the application of BSFF and the diverse cropping stimulated bacterial decomposition and clearly enhanced bacterivore nematodes, implying the stimulation of bacterial activity and nutrient cycling and potentially contributing to better soil health and pest suppression. Numerous studies have also reported that bacterivores and fungivores dominate in soils amended with organic inputs (Akhtar and Mahmood, 1996; Kimpinski et al., 2003; Liu et al., 2020; Lu et al., 2020; Rahman et al., 2014). Atandi et al. (2022) reported that amending the soil with organic inputs, enhanced the abundance of bacterivore nematodes in the organic systems compared to the conventional and control systems. Similar findings were reported by Kimpinski et al. (2003), in their 7-year study, where they found that compost and manure soil amendments enhanced the densities of bacterial-feeding nematodes in the potato roots rhizosphere.

Omnivores and predatory nematodes were present, although they were generally low across farming systems during the study period. Their occurrence have been well-documented in other studies and have been recorded to significantly reduce PPNs levels in the soil (Askary and Abd-Elgawad, 2017; Atandi et al., 2022; Kanwar et al., 2021; Khan and Kim, 2007). The presence of organic matter can support populations of natural enemies, such as predatory insects or nematodes, which actively prey on soil-dwelling pests (Stirling, 2014a). Enhanced soil organic matter in soils indirectly inhibit pest damage through creating microenvironments that are less favorable for the survival and reproduction of pests and supportive of natural enemies (Yardim and Edwards, 2003). Organic amendments enhance soil fertility and nutrient content. This alteration in soil composition can influence plant health, making them less susceptible to pests (Anedo et al., 2025; Delitte et al., 2021). Similarly, improving the nutritional status of plants results in healthier plants that are more tolerant to herbivores and nematodes (Herms, 2002).

This current study demonstrates that integrating cropping systems and insect frass, exert great influence on nematode abundance and community dynamics. Field results demonstrate that this combined approach not only suppresses harmful nematode populations but also enhances the abundance and diversity of beneficial free-living nematodes, particularly bacterivores and fungivores. Overall, these findings highlight the potential of integrating organic amendments such as BSFF with companion cropping systems as sustainable, environmentally friendly strategies for managing plant-parasitic nematodes while enhancing free-living ones in vegetable-cereal systems (VIPP) in Kenyan agroecosystems. Further studies are, however, required to establish the long-term effects of vegetable intensified push-pull systems and BSFF farming on the build-up or reduction of important free-living nematodes. Future work should quantify the long-term effects of these practices on crop yields, soil microbial communities, and other soil borne pests. Integrating molecular and chemical analysis techniques could also reveal deeper insights into functional changes within the nematode community and associated microbes.