The main aim of this work was to evaluate the impact of foraging behaviour of different nematode species on the dissemination of C. fumosorosea conidia and blastospores. The three different nematode species employed in these experiments were chosen specifically to compare all three foraging strategies, namely cruiser, intermediate and ambusher, and their effect. Additionally, a focus was put on the importance of the nematode’s 2nd stage cuticle and its role in spore dissemination and to achieve these insights, experiments were conducted with en- and ex-sheathed nematodes. Lastly, methods to enhance spore dissemination were explored, specifically by testing various adhesives which were used as an application medium.
4.1. Glass tube Experiment
The main intention of the glass tube experiment was to gain understanding of the influence of nematode foraging behaviour on spore dissemination in the conditions close to the soil environment. Generally, it was found that the application of nematodes in conjunction with fungal spores enhances their dispersal through the soil environment of the glass tube. Increased transmission, in comparison to the control with only spore application, was found with all three nematode strains. Steinernema carpocapsae overall exhibited the lowest number of CFUs transmitted, which may be attributed to their ambushing behaviour. Since ambushers are mainly found near the soil surface and wait for hosts to pass by (Campbell and Kaya 2000; Lewis et al. 1993), it can be assumed that this behaviour plays a role in their lower ability to disperse spores in this experimental design.
On the other hand, H. bacteriophora were discovered to be best at transferring spores over longer distances, as illustrated by the highest yield of CFUs in the third part of the glass tubes. We assume that H. bacteriophora’s capability to disseminate spores even far down the glass tubes occurs due to their cruising behaviour. Since cruisers are very active and don’t await host cues, they can be found throughout the soil profile (Lewis et al. 1993), which is in accordance with the observations mentioned here.
S. feltiae show an intermediary foraging behaviour that lies between ambusher and cruiser; therefore, they are active, but also remain stationary when no potential hosts are close by (Abd-Elgawad et al. 2017). Within this experiment, this behaviour in fact translates to the strain dispersing the highest number of CFUs extracted from soil of the centre part of the glass tubes. Though, dispersal to the third part was found to be lower than that of H. bacteriophora, yet still higher than S. carpocapsae.
Analysis of the experimental data also yielded information about the spore type that is best transmitted. It was concluded previously that conidia are generally better dispersed through a soil environment with the aid of nematodes than blastospores (Nermuť et al. 2020). The general trends regarding number of CFUs corresponding to nematode strain described before still hold true when solely examining transmission of spore type. Reasons as to why conidia are better dispersed are not yet determined since research in this direction is still lacking, however their tolerance against environmental stresses as well as their hydrophobicity could be of importance (Boucias et al. 1988; Humber 2008).
Another important finding is the significance of the nematode’s 2nd stage cuticle for dissemination of fungal spores. Data clearly shows that nematodes, regardless of strain or foraging behaviour, disperse less spores if they have been stripped of their 2nd stage cuticle. A nematode’s 2nd stage sheath is commonly more corrugated than the 3rd stage sheath, which may explain why spores are able to better adhere to the 2nd stage cuticle (Timper and Kaya 1989).
In general, the method used in this experiment still leaves room for improvement and alterations. For example, the wetness of the soil or usage of sterilised or unsterilised soil may influence dispersal rates.
4.2. Soil column experiment
The second experiment’s focus was the evaluation of different adhesives and how they influence spore dissemination. Furthermore, the hypotheses already explored in the first experimental series were assessed here as well.
. Surprisingly S. carpocapsae IJs were deemed to be the best dispersers of both spore types, which may be due to their foraging strategy being better suited for the experimental design (Campbell and Kaya 2000; Lewis et al. 1993). This contrasts with the previous experiment where S. carpocapsae performed considerably worse than both S. feltiae and H. bacteriophora. The difference could be explained by the diverse soil conditions that were simulated in the two experiments. While the glass tube experiment better replicates the conditions deeper within the soil, the conditions in the soil column experiment are more similar to those on the soil surface. As mentioned previously, nematodes employing the ambushing strategy, like S. carpocapsae, are predominantly found near the soil surface where they await hosts (Campbell and Kaya 2000). Furthermore, Wilson et al. (2012) suggests that S. carpocapsae is adapted to life in media other than mineral soils (e.g., peat, leaf litter or woody environments) where it can act as actively moving cruiser. The same brown soil was used in both our experimental arenas, but obviously, the design of the soil column experiment allowed cruising behaviour of S. carpocapsae.
Therefore, the soil column experiment appears to be better suited to the natural foraging behaviour of S. carpocasae. This aligns with the findings regarding increased spore dissemination of this species in the second experiment.
Additionally, the 2nd stage cuticle’s effect was once again confirmed through this experiment. Ex-sheathed nematodes, irrespective of strain, transmitted only very low numbers of spores; especially blastospore dispersal decreased significantly. Both experiments therefore point to the sheath as a crucial component for fungal spore transmission that should not be overlooked, particularly in pest management where spore dispersal enhancement is desired (Timper and Kaya 1989). Nevertheless, more research should be conducted to further examine the underlying mechanism of spore adhesion to nematode’s 2nd stage cuticles.
Lastly, the novel approach of using adhesives to enhance spore dissemination yielded promising results. Specifically sunflower seed oil applied as a medium with en-sheathed IJs seems to noticeably increase the number of CFUs detected for all three nematode species. It’s advantage mainly presented itself in the dissemination of conidia, since it appears that blastospore transmission is negligible regardless to adhesives used. A possible explanation for this is the hydrophilicity that presents a main characteristic of blastospores (Humber 2008). Moreover, blastospores have a lower tendency to withstand environmental stresses, when compared to conidia, therefore possibly lowering dispersal rates (Vega et al. 1999). Alternatively, blastospores may simply not be able to adhere to nematode’s 2nd stage cuticle as efficiently as conidia.
Tap water has been determined to be by far the weakest medium when compared to sunflower seed oil and liquid alginate. Even when conidia are applied with en-sheathed IJs only low dispersal rates were recorded, except for S. carpocapsae which still yielded considerable, albeit lower rate of spore distribution. Unexpectedly, the combination of S. carpocapsae with liquid alginate for the transmission of conidia exhibited the highest number of CFUs recorded within the scope of this experiment.
Overall, the use of adhesives has the potential to greatly influence the efficacy of spore dispersal by nematodes and may present an interesting opportunity for the use in integrated pest management and biocontrol. Sunflower seed oil appears to yield the best results irrespective of nematode strain and therefore might be the most promising candidate for conducting supplementary research and further improve spore dispersal.