Temperature-dependent modelling and spatial prediction reveal suitable geographical areas for deployment of two Metarhizium anisopliae isolates for Tuta absoluta management

Abstract

Tuta absoluta is one of the most devastating pests of Solanaceae crops in Africa. We previously demonstrated the efficacy of Metarhizium anisopliae isolates ICIPE 18, ICIPE 20 and ICIPE 665 against adult T. absoluta. However, adequate strain selection and accurate spatial prediction are fundamental to optimize their efficacy and formulations before field deployment. This study therefore assessed the thermotolerance, conidial yield and virulence (between 15 and 35 °C) of these potent isolates. Over 90% of conidia germinated at 20, 25 and 30 °C while no germination occurred at 15 °C. Growth of the three isolates occurred at all temperatures, but was slower at 15, 33 and 35 °C as compared to 20, 25 and 30 °C. Optimum temperatures for mycelial growth and spore production were 30 and 25 °C, respectively. Furthermore, ICIPE 18 produced higher amount of spores than ICIPE 20 and ICIPE 665. The highest mortality occurred at 30 °C for all the three isolates, while the LT₅₀ values of ICIPE 18 and ICIPE 20 were significantly lower at 25 and 30 °C compared to those of ICIPE 665. Subsequently, several nonlinear equations were fitted to the mortality data to model the virulence of ICIPE 18 and ICIPE 20 against adult T. absoluta using the Entomopathogenic Fungi Application (EPFA) software. Spatial prediction revealed suitable locations for ICIPE 18 and ICIPE 20 deployment against T. absoluta in Kenya, Tanzania and Uganda. Our findings suggest that ICIPE 18 and ICIPE 20 could be considered as effective candidate biopesticides for an improved T. absoluta management based on temperature and location-specific approach.

Figure 1

Effect of temperature on conidial germination of the three Metarhizium anisopliae fungal isolates ICIPE 18, 20 and 665. Error bars indicate the standard error of the mean at 95% CI (n = 72; N = 4; Tukey’s HSD test).

Figure 2

Relationship between temperature and radial growth rate of the three Metarhizium anisopliae fungal isolates. (A) Relative growth rates of Metarhizium anisopliae isolates ICIPE 18, ICIPE 20 and ICIPE 665 on SDA medium between 15 and 35 °C. Linear (dashed lines) and Brière-1 (continuous lines) nonlinear models fitted to observed values of the radial growth rate of Metarhizium anisopliae isolates (B) ICIPE 18, (C) ICIPE 20 and (D) ICIPE 665 at constant temperatures (n = 72; N = 4; Tukey’s HSD test).

Figure 3

Effect of temperature on conidial production of the three Metarhizium anisopliae isolates ICIPE 18, 20 and 665. Error bars indicate the standard error of the mean at 95% CI (n = 72; N = 4; Tukey’s HSD test).

Figure 4

Effect of temperatures on virulence of the three isolates of Metarhizium anisopliae against adult Tuta absoluta, 12 days post-inoculation. Different lowercase letters show significant difference (GLM, P ≤ 0.05) in mortality among fungal isolates across the different temperature regimes. Different uppercase letters denote significant difference (GLM, P ≤ 0.05) in mortality across the different temperature regimes for each isolate (n = 60; N = 4; Tukey’s HSD test).

Figure 5

Observed and predicted mortality of adult Tuta absoluta by Metarhizium anisopliae isolates ICIPE 18, ICIPE 20 and ICIPE 665 in relation to temperature using the linear and nonlinear models. The blue dots on the graph represent the cumulative values of the proportion of adult Tuta absoluta that were killed by the three isolates during the experiments at the respective temperatures. The curve represents the Logan-4 and Logan-1 nonlinear models that best fits the experimental data points and were used to predict the level of efficacy of isolates (A) ICIPE 18, (B) ICIPE 20 and (C) ICIPE 665 against adult Tuta absoluta. Fitted models are the dashed straight lines for linear regression and solid lines for the Logan models. Dashed lines above and below represent the upper and lower 95% confidence interval (n = 60; N = 4; Tukey’s HSD test).

Figure 6

Mass-production indices of Metarhizium anisopliae isolates ICIPE 18, ICIPE 20 and ICIPE 665. (A) Mean weight of conidia powder/kg of rice. (B) Mean number of conidia/g of powder. (C) Mean number of conidia/kg of powder. (D) Water content (%) of conidia. (E) Percentage conidial germination and (F) Percentage consumed substrate. Different lowercase letters above error bars indicate a significant difference across the treatments (n = 18; N = 6; Tukey’s HSD test). Middle quartile (line that divides the box into two parts) shows midpoint of the data. Middle box represents 50% of the scores for each treatment and the middle 50% values fall within the inter-quartile range.

Figure 7

Spatial patterns of predicted virulence of Metarhizium anisopliae isolates ICIPE 18 and ICIPE 20 against adult Tuta absoluta in Kenya: (A) ICIPE 18 and (B) ICIPE 20. The dots in green indicate Tuta absoluta records in the three countries. The figures were generated using the QGIS 3.10.2 software (https://qgis.org/downloads/).

Figure 8

Spatial patterns of predicted virulence of Metarhizium anisopliae isolates ICIPE 18 and ICIPE 20 against adult Tuta absoluta in Tanzania: (A) ICIPE 18 and (B) ICIPE 20. The dots in green indicate Tuta absoluta records in the three countries. The figures were generated using the QGIS 3.10.2 software (https://qgis.org/downloads/).

Figure 9

Spatial patterns of predicted virulence of Metarhizium anisopliae isolates ICIPE 18 and ICIPE 20 against adult Tuta absoluta in Uganda: (A) ICIPE 18 and (B) ICIPE 20. The dots in green indicate Tuta absoluta records in the three countries. The figures were generated using the QGIS 3.10.2 software (https://qgis.org/downloads/).