Experimental and Applied Acarology

, Volume 46, Issue 1, pp 275–285

Effect of temperature on virulence of Beauveria bassiana and Metarhizium anisopliae isolates to Tetranychus evansi

Authors

  • David M. Bugeme
    • International Centre of Insect Physiology and Ecology (ICIPE)
    • Department of BotanyJomo Kenyatta University of Agriculture and Technology (JKUAT)
    • International Centre of Insect Physiology and Ecology (ICIPE)
  • Markus Knapp
    • International Centre of Insect Physiology and Ecology (ICIPE)
  • Hamadi I. Boga
    • Department of BotanyJomo Kenyatta University of Agriculture and Technology (JKUAT)
Article

DOI: 10.1007/s10493-008-9179-1

Cite this article as:
Bugeme, D.M., Maniania, N.K., Knapp, M. et al. Exp Appl Acarol (2008) 46: 275. doi:10.1007/s10493-008-9179-1

Abstract

The virulence of three isolates of Beauveria bassiana (Bals.) Vuill. and 23 isolates of Metarhizium anisopliae (Metschnik.) Sorok. (Ascomycota: Hypocreales) against the tomato spider mite, Tetranychus evansi Baker and Pritchard (Acari: Tetranychidae), was assessed in the laboratory. The effect of temperature on germination, radial growth and virulence of selected isolates (two isolates of B. bassiana and nine of M. anisopliae) on T. evansi was also investigated in the laboratory. All the fungal isolates tested were pathogenic to the adult females of T. evansi, and there were significant differences in mortality between fungal isolates. The lethal time to 50% mortality (LT50) values ranged from 4.2 to 8.1 days and the LT90 values from 5.6 to 15.1 days. Temperature had significant effects on germination, radial growth and virulence of the various isolates. The best fungal germination was observed at 25 and 30°C, while for the fungal radial growth it was 30°C. All the isolates germinated and grew at all temperatures, but germination and radial growth varied with isolate and temperature. The selected isolates were all virulent to T. evansi, but virulence varied also with isolate and temperature.

Keywords

AscomycotaBeauveria bassianaHypocrealesGerminationMetarhizium anisopliaePathogenicityRadial growthTetranychidaeTetranychus evansiVirulence

Introduction

The spider mite, Tetranychus evansi Baker & Pritchard (Acari: Tetranychidae), is an exotic mite species, probably of South American origin (Gutierrez and Etienne 1986) and was first recorded in Africa in 1979 in Zimbabwe (Blair 1983), from where it spread northwards. More recently, it was also reported from central, western and northern Africa (Bonato 1999; Kreiter et al. 2002; Duverney et al. 2005). It is a pest of solanaceous crops, especially tomatoes (Silva 1954; Ramalho and Flechtmann 1979; de Moraes et al. 1986). If left uncontrolled under hot and dry conditions, T. evansi can destroy tomato plants within 3-5 weeks, and the farmer can lose his production within a week’s time. For instance, yield losses of up to 90% have been reported in Zimbabwe (Saunyama and Knapp 2003).

Due to problems related to the use of synthetic acaricides in controlling T. evansi (spider mite resistance and environmental contamination), the control of this pest is still a major problem for farmers and attracts a strong attention in the world of researchers. Thus, non-chemical control measures are being developed at the International Centre of Insect Physiology and Ecology (ICIPE) as alternatives to synthetic acaricides for the control of T. evansi. They include improved crop management, screening for resistance in commercial and wild tomato germplasm and biological control using predatory mites and entomopathogenic fungi.

Biological control of T. evansi with the predatory mites, Neoseiulus californicus McGregor and Phytoseiulus persimilis Athias-Henriot, was not effective (de Moraes and McMurtry 1986; Escudero and Ferragut 2005); however, a recently discovered strain of P. longipes Evans has shown promising results in laboratory experiments (Furtado et al. 2007). The virulence of entomopathogenic fungi including Beauveria bassiana (Bals.) Vuill. and Metarhizium anisopliae (Metsch.) Sorok. (Ascomycota: Hypocreales) towards spider mite species have been reported by many authors (Chandler et al. 2000; Barreto et al. 2004; Wekesa et al. 2005).

However, entomopathogenic fungi are exposed to a number of biotic and abiotic factors. Temperature, humidity and solar radiation are probably the most important environmental factors affecting survival and capability to cause mortality by entomopathogenic fungi (Benz 1987; Inglis et al. 2001). Temperature affects the pathogen-its germination, growth, survival and virulence- the host and the host-pathogen interaction (Ekesi et al. 1999; Dimbi et al. 2004; Kiewnick 2006). In general, optimum temperatures for germination, growth, sporulation and virulence of entomopathogenic fungi have been reported to range between 20 and 30°C (Ekesi et al. 1999; Tefera and Pringle 2003; Dimbi et al. 2004; Kiewnick 2006). Since variation in temperature tolerance among isolates can be significant (Ekesi et al. 1999; Tefera and Pringle 2003; Dimbi et al. 2004), this study was therefore initiated to evaluate (i) the virulence of B. bassiana and M. anisopliae isolates to T. evansi and (ii) to test the effect of temperatures on germination, radial growth and virulence of the selected isolates to T. evansi in order to pick isolates with a broad temperature range for further studies.

Materials and methods

Mite cultures

A stock culture of T. evansi was established in the laboratory at the ICIPE Headquarters, Nairobi, Kenya. Tetranychus evansi was reared on tomato, Lycopersicon esculentum Mill. variety Cal-J at 26 ± 2°C, 60–70% RH and a 12:12 L:D photoperiod. The initial culture originated from mites collected from tomato plants at Mwea Irrigation Scheme, Kenya, in 2001. Quiescent deutonymphs were collected from the mite culture using a fine camel hair brush and placed on tomato leaf discs. Two days later, newly emerged adult female mites were selected and used in the experiments.

Fungal cultures and viability assessments

The 26 fungal isolates used in this experiment were obtained from the ICIPE Arthropod Germplasm Centre (Table 1). Conidia were harvested by scraping the surface of 3-week-old sporulating cultures grown on Sabouraud dextrose agar (SDA) in Petri dishes at 26 ± 2°C. Conidia were suspended in 20 ml sterile distilled water containing 0.05% Triton X-100. The suspension was vortexed for 5 minutes to produce homogenous conidial suspension. The viability of conidia was then determined by spread-plating 0.1 ml of the suspension (titrated to 3.0 × 106 conidia ml−1) on SDA plates. A sterile microscope cover slip was placed on each plate. Plates were incubated at 26 ± 2°C and the percentage germination was determined from 100-spores for each plate using a compound microscope at 400 X magnification.

Bioassays

Virulence tests

The virulence of fungal isolates against T. evansi was tested by spraying 10 ml of a standard concentration of 1.0 × 107 conidia ml−1 on both sides of tomato leaf discs (25 mm diameter) using the Burgerjon’s spray tower (Burgerjon 1956) (INRA, Dijon, France), corresponding to 3.8 × 106 conidia cm−2. In the control treatments, leaf discs were sprayed with sterile distilled water containing 0.05% Triton X-100. The leaf discs were then air-dried under the laminar flow cabinet for 20 min and placed on wet cotton wool in Petri dishes. Twenty 1–2-day-old adult female T. evansi were then placed onto each of the treated tomato leaf discs. Mites were maintained in an incubator at 25 ± 2°C and transferred onto untreated leaf discs after 4 days. Mortality was recorded daily for 10 days. Dead mites were transferred to Petri dishes lined with moist filter paper to allow the growth of fungus on the surface of the cadaver. Mortality caused by fungus was confirmed by microscopic examination. Treatments were arranged in complete randomized blocks and replicated six times. The same experimental procedure was used in the experiments on the effect of temperature on the virulence of fungal isolates against T. evansi with the only difference that mites were maintained at various temperatures (20, 25, 30 and 35°C) and the treatment was replicated four times.

Effect of temperature on germination of fungi

Fungal isolates (11) that caused mortalities >70% within 10 days (LT90) were selected for this study. Conidial suspension (0.1 ml) of 3 × 106 conidia ml−1 was spread on SDA plates. A sterile microscope cover slip was randomly placed on each plate. Plates were sealed with Parafilm M and incubated at 20, 25, 30 and 35°C in complete darkness. At 24 h post-inoculation, 1 ml formaldehyde (0.5%) was transferred onto each plate to halt germination. Percentage germination was then determined from 100-spores for each plate at 400× magnification. Treatments were arranged in complete randomized blocks and replicated four times.

Effect of temperature on radial growth

This study was also carried out on the 11 selected fungal isolates during the virulence bioassay. Conidial suspension of 1 × 107 conidia ml−1 was spread-plated on SDA plates, which were then incubated at 26 ± 2°C for 3 days in order to obtain mycelial mats. The unsporulated mycelial mats were cut from culture plates into round agar plugs using an 8-mm diameter cork borer (Rapilly 1968). Each agar plug was then transferred singly onto the centre of a fresh SDA plate. Plates were sealed with Parafilm M and incubated in complete darkness at 20, 25, 30 and 35°C. Radial growth was then recorded daily for 10 days by measuring colony cardinal diameters, through two orthogonal axes previously drawn on the bottom of each Petri dish to serve as a reference, using a simple plastic ruler. The experiment was replicated four times.

Statistical analysis

Mortality data were corrected for natural mortality in the controls (Abbott 1925) and arcsine-transformed to normalize the data before analysis of variance (ANOVA) (SAS Institute 1999–2001). Means were separated by Student-Newman-Keuls test at = 0.05. Lethal time to 50% mortality (LT50) and the lethal time to 90% mortality (LT90) values were estimated with repeated measures logistic regression using generalised estimating equations (GEE) (Stokes et al. 2000). All analyses were carried out using the GENMOD procedure of SAS (SAS Institute 1999–2001). Data on germination were also arcsine-transformed before ANOVA. Radial growth data were also subjected to analysis for a completely randomised design using the ANOVA procedure of SAS (SAS Institute 1999–2001).

Results

Virulence of fungal isolates

In the viability tests, 86.9 ± 1.0 to 96.3 ± 0.7% of spores germinated. Mean mortality in the control was 12.9 ± 0.9% ten days after treatment. All the 26 fungal isolates tested were pathogenic to adult females of T. evansi. Mortality caused by B. bassiana was not significantly different between the isolates. However, there was a significant difference in mortality between isolates of M. anisopliae (F26,135 = 20.49; P < 0.0001) (Table 1). The LT50 values ranged from 4.2 to 8.1 days and the LT90 values from 5.6 to 15.1 days, with B. bassiana isolate ICIPE279 having the shortest lethal time values of 4.2 (LT50) and 5.6 days (LT90) (Table 1). Based on these results, 11 fungal isolates were selected for further studies.
Table 1

Pathogenicity of Beauveria bassiana and Metarhizium anisopliae isolates against Tetranychus evansi

Species/Isolates

Year of isolation, Host/Substrate

Percent mortality ± SE

LT50 (days) (95% fiducial limits)

LT90 (days) (95% fiducial limits)

Beauveria bassiana

ICIPE279

1996, Soil

95.2 ± 2.3a

4.2 (3.6–4.9)

5.6 (4.8–6.4)

ICIPE273

2004, Soil

83.3 ± 5.6abcd

6.0 (5.3–7.9)

7.5 (6.4–8.7)

ICIPE278

2005, Cyclocephala sp.

83.0 ± 7.7abcd

5.0 (3.3–7.7)

7.1 (4.6–10.9)

Metarhizium anisopliae

ICIPE24

1999, Soil

90.5 ± 3.8ab

6.3 (5.8–6.8)

7.3 (6.6–8.1)

ICIPE84

2003, Onitacris turbiddacavroisi

89.4 ± 3.7abc

6.5 (6.0–6.9)

7.5 (7.0–8.0)

ICIPE78

1990, Temnoschoita nigroplagiata

86.8 ± 8.8abc

5.7 (4.6–7.0)

7.6 (6.1–9.5)

ICIPE43

2005, Soil

84.8 ± 5.7abcd

6.0 (4.9–7.4)

8.0 (6.7–9.6)

ICIPE55

2005, Soil

81.7 ± 9.2abcd

5.6 (4.5–6.9)

7.4 (5.7–9.7)

ICIPE59

2005, Caterpillar

79.9 ± 7.1abcd

6.0 (6.4–7.6)

8.3 (7.4–9.2)

ICIPE8

1990, Galleria mellonella

79.3 ± 8.8abcd

6.5 (4.6–7.0)

7.9 (6.5–9.5)

ICIPE51

2005, Soil

73.0 ± 7.2abcde

7.5 (6.6–8.4)

9.4 (8.3–10.6)

ICIPE7

1996, Amblyomma variegatum

70.9 ± 6.3abcdef

7.7 (6.6–9.0)

9.8 (8.2–11.7)

ICIPE25

1999, Sandy Soil

68.4 ± 8.0abcdef

7.2 (6.6–7.9)

8.9 (7.7–10.1)

ICIPE48

2005, Unknown

60.1 ± 7.9bcdefg

7.6 (6.6–8.8)

10.7 (9.1–12.6)

ICIPE49

2005, Soil

57.9 ± 9.0cdefg

7.9 (4.0–15.5)

12.9 (7.0–23.8)

ICIPE315

2005, Tetranychus urticae

54.6 ± 6.3defg

7.7 (6.3–9.4)

15.1 (10.8–21.0)

ICIPE316

2005, Tetranychus spp.

54.0 ± 10.5defg

8.1 (6.6–9.9)

11.2 (8.7–14.3)

ICIPE95

2005, Soil

44.3 ± 11.5efg

ICIPE62

1990, Soil

43.8 ± 10.4efg

ICIPE21

1999, Lacusta gregaria

43.7 ± 8.2efg

ICIPE30

1989, Busseola fusca

42.4 ± 12.5efg

ICIPE41

1990, Soil

40.6 ± 3.5fg

ICIPE18

1989, Soil

37.0 ± 9.4gh

ICIPE97

2005, Unknown

35.8 ± 7.3gh

ICIPE69

1990, Soil

35.0 ± 4.3gh

ICIPE20

1989, Soil

30.4 ± 4.5gh

Control

12.9 ± 0.9h

Percent mortality, LT50 and LT90 values at 25 ± 2ºC

Means followed by the same letter are not significantly different (Student-Newman-Keuls test, P > 0.05)

Effect of temperature on germination of fungal isolates

The germination for all isolates was above 65% at the four temperatures, except at 35°C where germination was low in B. bassiana isolates ICIPE279 and ICIPE278. Germination values ranged from 65.8% to 86.3%, from 74.9% to 95.0%, from 81.7% to 96.8% and from 15.1% to 85.6% at 20, 25, 30 and 35°C, respectively. Significant differences in germination between fungal isolates were observed at 20°C (F10,33 = 9.18; P < 0.0001), 25°C (F10,33 = 28.16; P < 0.0001), 30°C (F10,33 = 7.92; P < 0.0001) and 35°C (F10,33 = 284.49; P < 0.0001).

Effect of temperature on radial growth

As in the case of germination, there were significant differences in radial growth between fungal isolates at 20°C (F10,33 = 31.28; P < 0.0001), 25°C (F10,33 = 14.22; P < 0.0001), 30°C (F10,33 = 21.76; P < 0.0001) and 35°C (F10,33 = 39.69; P < 0.0001). Fungal isolates grew at all temperatures but for most isolates, the growth was slower at 20 and 35°C, than at 25 and 30°C. The two isolates of B. bassiana recorded the least radial growth at all temperatures. Fungal radial growth varied from 0.6 to 2.1, from 1.2 to 2.4, from 1.2 to 4.4 and from 0.7 to 2.3 mm day−1 at 20, 25, 30 and 35°C, respectively.

Effect of temperature on virulence

Mortality in the controls did not exceed 7.5%, except at 35°C where 16.3% mortality was recorded. The 11 fungal isolates tested were pathogenic to the tomato spider mite at all temperatures; however, mortality varied with fungal isolate and temperature (Table 2). For instance, significant differences in mortalities were observed between fungal isolates at 20 (F10,33 = 2.43; P = 0.0267) and 25°C (F10,33 = 2.62; P = 0.0181). There was no significant difference in mortality between fungal isolates at 30 (F10,33 = 0.98; P = 0.4762) and 35°C (F10,33 = 1.15; P = 0.3607) (Table 2). Among the fungal isolates, only B. bassiana isolate ICIPE279 was virulent across temperatures followed by B. bassiana isolate ICIPE278 and M. anisopliae isolate ICIPE7 (Table 2). The LT50 values ranged from 6.8 to 24.6 days at 20°C, from 4.8 to 9.7 days at 25°C, from 2.6 to 5.8 days at 30°C and from 1.9 to 3.4 days at 35°C (Table 2). The LT90 values ranged from 9.0 to 35.3, from 6.7 to 11.6, from 3.3 to 7.2 and from 2.8 to 4.9 days at 20, 25, 30 and 35°C, respectively (Table 2).
Table 2

Effect of temperature on virulence of Beauveria bassiana and Metarhizium anisopliae to the tomato spider mite, Tetranychus evansi: Lethal time to 50% and 90% mortality

Species/isolates

20ºC

25ºC

% Mortality ± SE

LT50 (95% fiducial limits)

LT90 (95% fiducial limits)

% Mortality SE

LT50 (95% fiducial limits)

LT90 (95% fiducial limits)

B. bassiana

ICIPE279

72.5 ± 15.1aA

6.8 (5.3–8.6)

9.0 (6.6–12.3)

88.5 ± 4.3abA

7.5 (6.4–8.8)

8.7 (7.6–10.1)

ICIPE278

50.5 ± 15.8abB

9.2 (6.4–13.2)

13.4 (8.9–20.0)

90.4 ± 4.1abA

4.8 (3.0–7.6)

6.7 (4.4–10.2)

M. anisopliae

ICIPE55

26.3 ± 10.1abC

11.3 (10.5–12.3)

12.7 (11.8–13.7)

54.4 ± 9.8bB

9.7 (8.6–10.9)

12.5 (10.5–14.8)

ICIPE59

8.8 ± 2.4bB

24.6 (15.2–40.0)

35.3 (17.0–73.4)

75.0 ± 10.2bA

7.1 (6.0–8.4)

9.2 (7.5–11.2)

ICIPE78

50.9 ± 12.1abB

8.9 (6.1–13.0)

13.4 (8.4–21.3)

72.7 ± 7.8abAB

9.3 (8.9–9.7)

11.6 (10.3–13.0)

ICIPE84

37.8 ± 12.1abB

10.8 (8.7–13.4)

13.4 (10.3–17.5)

97.1 ± 1.7aA

6.2 (5.9–6.5)

7.7 (7.5–8.0)

ICIPE24

43.8 ± 6.6abB

11.1 (8.1–15.3)

18.8 (14.5–24.4)

77.3 ± 13.3abA

7.4 (6.4–8.6)

9.4 (7.5–11.8)

ICIPE25

21.3 ± 8.8abB

13.0 (10.2–16.4)

15.6 (11.6–20.8)

90.3 ± 6.2abA

6.5 (5.1–8.2)

8.1 (6.5–10.2)

ICIPE8

42.5 ± 10.6abC

10.5 (6.8–16.1)

15.9 (11.6–21.9)

66.5 ± 6.5abB

8.1 (7.1–9.2)

11.0 (9.3–13.1)

ICIPE43

30.4 ± 12.3abB

13.5 (11.4–15.9)

16.1 (13.1–19.7)

65.4 ± 12.3abA

8.8 (7.9–9.8)

10.5 (9.2–11.9)

ICIPE7

64.8 ± 13.4abB

7.4 (5.8–9.5)

10.0 (7.5–13.3)

83.7 ± 2.8abAB

6.8 (6.4–7.3)

8.3 (7.8–8.8)

 

30°C

35°C

% Mortality ± SE

LT50 (95% fiducial limits)

LT90 (95% fiducial limits)

% Mortality SE

LT50 (95% fiducial limits)

LT90 (95% fiducial limits)

B. bassiana

ICIPE279

100aA

4.1 (3.8–4.6)

5.0 (4.8–5.2)

98.5 ± 1.5aA

1.9 (1.0–3.5)

2.9 (1.6–5.1)

ICIPE278

98.5 ± 4.4aA

2.6 (2.1–3.2)

3.3 (2.7–4.1)

97.1 ± 1.7aA

2.0 (1.5–2.6)

2.8 (2.4–3.2)

M. anisopliae

ICIPE55

100aA

3.7 (3.0–4.6)

4.4 (3.4–5.8)

97.2 ± 1.6aA

2.6 (2.5–2.8)

3.0 (2.8–5.1)

ICIPE59

93.8 ± 4.4aA

5.8 (5.0–6.7)

7.2 (6.3–8.3)

91.5 ± 3.7aA

3.4 (3.0–3.9)

4.4 (3.8–5.1)

ICIPE78

100aA

4.4 (3.9–5.0)

5.2 (4.5–6.0)

100aA

2.7 (2.2–3.3)

3.3 (2.7–3.9)

ICIPE84

100aA

4.3 (3.8–4.8)

5.3 (4.6–6.0)

86.8 ± 11.4aA

2.9 (2.4–3.3)

3.4 (2.9–4.1)

ICIPE24

100aA

4.4 (3.9–4.9)

5.4 (4.8–6.0)

93.8 ± 4.4aA

2.8 (2.4–3.2)

3.3 (2.7–3.9)

ICIPE25

93.8 ± 6.3aA

5.2 (4.6–5.8)

6.5 (5.5–7.6)

100aA

3.0 (2.6–3.5)

3.6 (3.0–4.3)

ICIPE8

100aA

4.7 (4.0–5.5)

5.8 (5.3–6.4)

100aA

3.0 (2.3–3.8)

3.9 (3.1–5.0)

ICIPE43

98.6 ± 1.4aA

3.5 (3.0–4.0)

4.4 (3.8–5.1)

100aA

2.9 (2.8–3.1)

3.2 (3.1–3.3)

ICIPE7

98.6 ± 1.4aA

3.5 (2.5–4.9)

4.3 (3.0–6.2)

91.4 ± 5.0aAB

3.2 (1.9–5.5)

4.9 (3.4–7.0)

Means (± SE) within within one temperature (comparing 11 isolates) followed by the same lower case letter, and within one isolate (comparing 4 temperatures) followed by the same upper case letter are not significantly different (Student-Newman-Keuls test, P > 0.05)

Discussion

Out of 26 fungal isolates tested in the present study, 13 isolates were previously tested against T. evansi (Wekesa et al. 2005) and were all pathogenic to the mite; but there was significant variation between the isolates. Variation in the pathogenic activity between the 26 fungal isolates against T. evansi was also observed in the present study. Intraspecific differences in the pathogenicity of the mitosporic fungi B. bassiana,M. anisopliae,Hirsutella thompsonii Fisher and Lecanicillium lecanii complex have also been reported in other mite species (Tamai et al. 2002; Barreto et al. 2004; Alves et al. 2005; Brooks and Wall 2005; Chandler et al. 2005).

It is generally admitted that the most virulent fungal isolates are the ones isolated from the host. This is true in some cases as the one reported by Pĕna et al. (1996) where fungal isolates that originated from the mite Polyphagotarsonemus latus Banks were more virulent to this species than those isolated from other hosts. However, this was not the case in our study where the virulent isolates did not originate from spider mite species. The two fungal isolates isolated from Tetranychus species (ICIPE315 and ICIPE316) were not virulent to T. evansi. Wekesa et al. (2005) reported similar results with the same mite species. The virulence of fungal isolates from non-Acari hosts to Acari hosts have also been reported elsewhere (Kaaya et al. 1996; Samish et al. 2001; Shaw et al. 2002).

The best temperature for germination of the 11 selected fungal isolates was between 25 and 30°C, which is in agreement with other published reports (Ekesi et al. 1999; Tefera and Pringle 2003; Dimbi et al. 2004; Kiewnick 2006). However, germination of B. bassiana isolates was low at 35°C, compared to those of M. anisopliae isolates. Although all the fungal isolates grew at all the temperatures, it appeared that the favourable temperature for most isolates was 30°C. Similar results were reported by Tefera and Pringle (2003). However, Ekesi et al. (1999) and Dimbi et al. (2004) reported that the optimum temperature for radial growth of most isolates of B. bassiana and M. anisopliae was 25 and 30°C. Ouedraogo et al. (1997) reported that the optimum temperature for vegetative growth of M. anisopliae isolates ranged between 25 and 32°C, with 25°C being the optimum for most isolates. The growth rate of fungal isolates did not necessarily relate to virulence. For example, the B. bassiana isolate ICIPE279 that had a radial growth of 0.6 mm/day at 20°C caused 72.5% mortality in T. evansi while the M. anisopliae isolate ICIPE55 with a radial growth of 2.1 mm/day induced only 26.3% mortality at the same temperature.

The infection of B. bassiana and M. anisopliae in T. evansi increased as temperature increased. Most fungal isolates were more (highly) virulent at 25, 30 and 35°C than at 20°C, which is in agreement with other published reports (Thomas and Jenkins 1997; Ekesi et al. 1999; Dimbi et al. 2004). For instance, Dimbi et al. (2004) reported that M. anisopliae isolates were more virulent to the three African tephritid fruit flies, Ceratitis capitata (Wiedemann), C. fasciventris (Bezzi) and C. cosyra (Walker) at 25, 30 and 35, than at 20°C. However, Ekesi et al. (1999) showed that some isolates of B. bassiana and M. anisopliae were highly virulent at 20°C when infecting the legume flower thrips, Megalurothrips sjostedti (Trybom).

The findings of this study highlight the importance of strain selection as stressed by Soper and Ward (1981). Beauveria bassiana isolates ICIPE279 and ICIPE278, and M. anisopliae isolates ICIPE78 and ICIPE7 were selected as candidates for control of T. evansi because of their ability to infect and cause a high rate of mortality between 20 and 35°C, which is the range of temperature where the pest is found. However, further studies-such as the effect of these isolates on the various developmental stages of T. evansi, the effect of host plant on the virulence of the fungal isolate(s), the efficacy of these isolates in reducing T. evansi populations in screenhouse and field-need to be carried out in order to develop the isolates as biological control agents of T. evansi and as component of integrated spider mite management.

Acknowledgments

The authors are grateful to Drs. N. Jiang and F. Schultess (ICIPE) for reviewing the first draft of the manuscript. The authors are also grateful to A. Wanjoya for statistical advice and Ms. E.O. Ouna, Mr. R. Rotich, Mr. C. Kyallo and Mr. B. Muia for technical assistance. This study received financial support from the SII-Dutch government fund through the African Regional Postgraduate Programme in Insect Science (ARPPIS) of ICIPE and from the German Federal Ministry for Economic Cooperation and Development (BMZ).

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© Springer Science+Business Media B.V. 2008