Soil texture, infective juvenile concentration, and soil organic matter influence the efficacy of Steinernema feltiae isolate Lican Ray

Abstract

The influence of infective juveniles (IJs) concentration, soil texture, IJ-host distance and organic matter (OM) content, at different decomposition degree, on the activity of the nematode Steinernema feltiae isolate Lican Ray (LR) was examined using Galleria mellonella larvae. Bioassays were conducted in tubes of varied length, filled with soil of different textures, placed either vertically or horizontally. In the concentration assay, highest IJ concentrations caused maximum larval mortality in all soil types (440, 2,200 and 4,400 IJs in clay, loam and sandy loam). In the second assay, soil texture (loam, clay or sandy loam) interacted significantly with IJ-host distance (10, 20, 30, 40 cm, horizontally; 30, 50, 70 cm, vertically), and distances of 30 cm or more affected IJ effectiveness on the control of G. mellonella. The effect was stronger in clay and sandy loam than in loam soils, where IJ moved up to 40 cm horizontally and 70 cm vertically. In the third assay, OM content (0, 2, 4, 6 and 8%) and its decomposition degree (initial, medium and advanced) did not interact to influence IJ movement in all treatments that contained any percentage of OM (2-8%). Only in the soil with no OM, IJ did not cause death of larvae at all. These results show the potential of S. feltiae LR to be used in different soil textures, as long as the content of soil OM allows its dispersal and host infection, in order to optimize the pest-control activity of the nematode.

The influence of infective juveniles (IJs) concentration, soil texture, IJ-host distance and organic matter (OM) content, at different decomposition degree, on the activity of the nematode Steinernema feltiae isolate Lican Ray (LR) was examined using Galleria mellonella larvae. Bioassays were conducted in tubes of varied length, filled with soil of different textures, placed either vertically or horizontally. In the concentration assay, highest IJ concentrations caused maximum larval mortality in all soil types (440, 2,200 and 4,400 IJs in clay, loam and sandy loam). In the second assay, soil texture (loam, clay or sandy loam) interacted significantly with IJ-host distance (10, 20, 30, 40 cm, horizontally; 30, 50, 70 cm, vertically), and distances of 30 cm or more affected IJ effectiveness on the control of G. mellonella. The effect was stronger in clay and sandy loam than in loam soils, where IJ moved up to 40 cm horizontally and 70 cm vertically. In the third assay, OM content (0, 2, 4, 6 and 8%) and its decomposition degree (initial, medium and advanced) did not interact to influence IJ movement in all treatments that contained any percentage of OM (2–8%). Only in the soil with no OM, IJ did not cause death of larvae at all. These results show the potential of S. feltiae LR to be used in different soil textures, as long as the content of soil OM allows its dispersal and host infection, in order to optimize the pest-control activity of the nematode.

Figure 1:

Through the lateral opening of this t-shaped PVC matrix (A), a modified Petri dish containing soil and larvae is introduced. A removable lid with a metallic mesh for gas exchange covers the lateral opening (B). IJs are pipetted through the upper opening (C).

Figure 2:

Boxplot showing percentage of last instar larvae Galleria mellonella mortality at (I) three, (II) six and (III) nine DAI, inoculated with 440, 2,200 or 4,400 IJs/5 mL of Steinernema feltiae LR in clay (C), loam (L) or sandy loam (S) soil, corrected according to the Schneider-Orelli’s formula). In (I) bars with different capital letters indicate statistical differences for soil texture (Tukey’s test; p ≤ 0.05). In (II) and (III) bars with different lower-case letter indicates statistical differences for the interaction between IJ concentration and soil texture (Tukey’s test; p ≤ 0.05). In Figs. 2–4, boxes represent the third quartile (75th percentile), median (50th percentile), first quartile (25th percentile), and mean percentage (x) of five replicates, with upper whiskers reaching Q3 +1.5×interquartile range (IQR) and lower whiskers Q1−1.5 × IQR.

Figure 3:

Boxplot showing percentage of last instar larvae Galleria mellonella mortality at a distance of 10, 20, 30 or 40 cm horizontally from the inoculation point of IJ of Steinernema feltiae LR in clay (C), loam (L) or sandy loam (S) soil at (I) three, (II) six and (III) nine DAI. Bars with different letter in each evaluation time, indicate statistical differences, according to Tukey’s test (p ≤ 0.05).

Figure 4:

Boxplot showing percentage of last instar larvae Galleria mellonella mortality at a distance of 30, 50 or 70 cm vertically from the inoculation point of IJ of Steinernema feltiae LR in clay (C), loam (L) or sandy loam (S) soil at (I) three, six (II) and (III) nine DAI. Bars with different letter in each evaluation time, indicate statistical differences, according to Tukey’s test (p ≤ 0.05).

Figure 5:

Boxplots showing percentage of last instar larvae Galleria mellonella mortality caused by Steinernema feltiae LR in loam soil at two and four DAI with 0, 2, 4, 6 or 8% OM. Bars with different letters indicate statistical differences, according to Tukey’s test (p ≤ 0.05) (lower case for two DAI and capital letters for four DAI). In figs 5 and 6, boxes represent the third quartile (75th percentile), median (50th percentile), first quartile (25th percentile), and mean percentage (x) of 15 replicates, with upper whiskers reaching Q3 + 1.5 × interquartile range (IQR) and lower whiskers Q1−1.5 × IQR. Outliers beyond the boxes limits are plotted as individual points.

Figure 6:

Boxplots showing percentage of last instar larvae Galleria mellonella mortality caused by Steinernema feltiae LR in loam soil at two and four DAI with OM at different decomposition degrees (initial, medium and advanced) control soil (unknown) and soil with no OM (calcined). Bars with different letters on bars indicate statistical differences, according to Tukey’s test (p ≤ 0.05) (lower case for two DAI and capital letters for four DAI).