Previous research has shown that application of either
H. downesi or
S. carpocapsae can provide significant suppression of LPW below acceptable thresholds (Kapranas et al.
2017). Our trials at seven different sites over two years showed that
H. downesi gave slightly better results but not significantly different from
S. carpocapsae (Kapranas et al.
2017), in contrast to earlier work showing that
H. downesi was superior (Dillon et al.
2006; Williams et al.
2013a,
b). Typically EPN suspension is applied directly to the soil around the stump (Dillon et al
2006; Williams et al.
2013a; Kapranas et al.
2017), but in our study we tested a modified method of application by applying the suspension on the top edges of the stump, targeting the gap between the bark and the wood (though some of the suspension will eventually make it to soil by spill over the stump). In the latter method it is likely that some nematodes were more exposed to UV light and desiccation and consequently died but, on the other hand, most of the nematodes entered the space between the wood and the bark and would have transferred, at least to some extent passively, along the rootways, thus reaching more quickly the weevils that are typically found there.
Steinernema carpocapsae performed better when applied by drenching the suspension around the stump, rather than when applied on top of the stumps: parasitism rates tended to be higher, but not statistically significant. However, weevil suppression was greater, in particular with reference to weevil infestation thresholds set at 20 weevils per stump. In fact,
S. carpocapsae provided adequate control only when nematodes were drenched in the soil. One hypothesis to explain why
S. carpocapsae does better when applied to soil rather than at the top is that IJs that are carried into the space between the bark and wood may be encouraged to sit and wait there.
S. carpocapsae was detected in this location up to two years post-application, though this was assumed to include nematodes that had recycled in LPW (Harvey and Griffin,
2016). The host species with which
S. carpocapsae has most frequently been associated is the codling moth,
Cydia pomonella (Peters
1996). Codling moths spend most of their life cycle as larvae within fruits. Natural infections are of cocooned larvae located near the base of trees and close to the soil (Lacey et al.
2006). For a parasite targeting larvae moving from fruit to soil the ideal location to sit and wait would be the protected areas on the tree bark. Previous research, using
S. carpocapsae in a peaty-gley, clear-felled area has also shown that application of this species provides better control when the applied nematodes were immediately covered by soil (subsurface drench) than when applied to the soil surface (Brixey et al.
2006). On the other hand
H. downesi seems to be more efficacious in the ‘top’ vs. the ‘standard’ application. Specifically, weevil suppression with reference to the infestation threshold (20 weevils per stump) was greater when this species was applied on top in all three field trials, whereas adequate suppression when applied around the stump was observed in only one of them. These results can be explained by mutually non-exclusive hypotheses. Firstly, the space between bark and wood may provide suitable conditions for “cruise forager”
H. downesi to detect and move towards pine weevil volatiles. Secondly,
H. downesi might be more vulnerable to predation by collembolans, mites and nematophagous fungi found in the soil. For instance, in previous studies with
H. bacteriophora, poor persistence was positively correlated with numbers of mites and collembola in plots where nematodes were surface-applied, but not in plots where they were subsurface-applied (Wilson and Gaugler
2004). However, other studies suggested that EPN natural enemies such as mites and nematophagous fungi show stronger responses in the presence of steinernematids rather than heterorhabditids (Duncan et al.
2007; Greenwood et al.
2011). Lastly it is possible that nematodes applied on the top edges of the stump more easily reach other host species (such as
Pissodes spp.) that occur in between the wood and the bark, and that these hosts are more susceptible to
H. downesi, thus improving recycling post-application. Thus, application of the EPN suspension on top of the stump where a proportion is carried down between the bark and wood might be beneficial, even to an extent that offsets increased mortality of the portion of the suspension that is subject to the detriments of UV light and desiccation if it remains on the outside of the stump.
Parasitism rates of weevils declined both in relation to depth and distance from the bole,
which is in agreement with previous studies (Dillon et al.
2006; Williams et al.
2013a,
b; Kapranas et al.
2017). However, these rates of decline are further influenced by the method of application. Our results show that at, Cloondara, ‘top’ application of
H. downesi led to a steeper decline in parasitism in relation to distance from bole, suggesting that the nematodes stayed close to the bole and did not disperse to the more distant parts of the root system. Since most of the weevils at this site were close to the bole (ca. 72% of weevils were less than 10 cm distance from the bole compared to 49% at Doon and 51% at Tigroney), killing a high proportion in this region should result in higher parasitism overall and a drop off in percentage parasitism with distance might have not been important. However, application of nematode suspension on top of the stump can be more efficacious against the weevils that are found in the stump above ground level, as indicated by significant interaction of intervention and depth at Doon (Fig.
3c). Typically, only a small proportion of weevils (on average 10 %) is found above ground, but in sites with a higher proportion above ground a top application might be recommended. Kapranas et al. (
2017) showed that pine weevil distribution within stumps, as influenced by soil properties, can explain patterns of parasitism and suppression by EPN. The results of the current trials indicate that weevil distribution may also help explain the relative success of different application methods. Results of these trials represent a rare insight into how application method may influence parasitism and hence efficacy of EPN applied against a tree-boring pest.
In conclusion our results show that even simple adjustment in the application of EPN suspension around the stumps of pine trees can have significant effects on weevil suppression to below economic damage thresholds. However, increased weevil suppression resulting from alternative methods of application depends on EPN species. While for S. carpocapsae better results are always achieved when the suspension is applied directly on the soil around the stump, for H. downesi it appears that LPW suppression can be anticipated more frequently when the EPN suspension is applied in top of the stump. Spray workers could direct the application of EPN suspension either in close proximity to the soil or on the top edges of stumps depending on the species being used, thus enhancing the efficacy of the EPN suspension.