Commercial-scale field studies were conducted in citrus orchards to test the efficacy of entomopathogenic nematodes (EPNs) in controlling the soil-dwelling life stages of the key pest, Thaumatotibia ...leucotreta. Trials were conducted in three provinces of South Africa from 2011 to 2013. Two EPN species were tested: Heterorhabditis bacteriophora and Steinernema feltiae. Both EPN species were applied underneath citrus tree canopies, either with a spray machine or through the microsprinkler irrigation system. The concentration of nematodes and frequency of application varied. Efficacy of these treatments was measured by evaluation of EPN infection of sentinel T. leucotreta larvae in the soil at various intervals after application and by T. leucotreta infestation of fruit. Successful control was recorded in several trials, with EPN infection of larvae frequently reaching 80 %. However, in a few trials, this level of infection was assisted by naturally occurring EPNs. Fruit infestation by T. leucotreta larvae was reduced by up to 88 %. Efficacy appeared to be limited by suboptimal soil moisture and temperature. The former was influenced mainly by mode and relative thoroughness of irrigation before and after EPN application. This was best obtained through microsprinkler application. Nematode species and concentration also influenced efficacy. Both EPN species survived and persisted in the soil of the treated orchards for up to 4 months. Promising results support their potential for use in an integrated pest management programme in citrus in South Africa.
•Thaumatotibia leucotreta is an important pest of citrus in sub-Saharan Africa.•Heterorhabditis bacteriophora and Steinernema feltiae are virulent to T. leucotreta.•In field trials, more than 80 % of T. leucotreta larvae were infected with nematodes.•Fruit infestation with T. leucotreta larvae was reduced by up to 88 %.•Best results were achieved with monthly applications of EPNs for four months.
Infection of codling moth, Cydia pomonella (L.), larvae by the entomopathogenic nematode, Heterorhabditis zealandica, was studied under semi-field conditions. Late-instar diapausing codling moth ...larvae, coccooned in perforated cardboard strips, were used in all trials to evaluate the nematode treatments involved. A morning application of 0.5, 1.0 and 1.5 MM infective juveniles (IJs) per tree resulted in 85%, 95% and 100% codling moth larval infection, respectively. Contrasting results were obtained for an evening application, with low levels of codling moth infection (<50%). Nematode treatment strips placed in direct sunlight resulted in almost no infection (<10%), opposed to strips attaining 67% infection placed in the shade.
Adequate moisture levels are required for nematode survival and subsequent efficacy as entomopathogens. Formulation of nematodes aimed at aboveground applications may assist in maintaining such ...moisture levels. In this study, we report the effects of a superabsorbent polymer formulation, Zeba® on the performance of an entomopathogenic nematode, Heterorhabditis zealandica Poinar, for controlling diapausing codling moth, Cydia pomonella (L.) larvae in cryptic habitats on trees. Water activity (a
w
-value) on bark was considered to be an indication of moisture levels on trees in cryptic habitats where codling moth larvae are known to occur, thereby influencing nematode efficacy. H. zealandica was only able to infect codling moth larvae at a
w
≥0.92, with a
w50
=0.94 and a
w90
=0.96. Laboratory experiments in which nematode concentration was investigated indicated a positive linear relationship between the concentration of nematodes applied and the level of control obtained, with the highest level of mortality recorded at 80 IJs/larva, requiring at least 4 h of conditions conducive to nematode activity to ensure infectivity and subsequent efficacy. Further experimentation showed that the use of the Zeba formulation, together with the nematodes, improved the level of control obtained at 60% and 80% RH in the laboratory and that it also enhanced the survival and infection-ability of the nematodes in the field. The study conclusively illustrates that the tested formulation assisted in maintaining adequate moisture levels on the application substratum, as required for nematode survival and subsequent efficacy.
The non-insecticidal control strategies currently being implemented in South African orchards for the control of codling moth, Cydia pomonella (L.) may be hampered by wooden fruit bins being infested ...with diapausing codling moth larvae, acting as a potential source of re-infestation. Key factors contributing to the success or failure of an entomopathogenic nematode application were investigated using the SF 41 isolate of Heterorhabditis zealandica in laboratory bioassays with wooden minibins. Under operational conditions, an application rate of 100 IJs/mL (LD90=102 IJs/mL) effectively controlled codling moth larvae in these bins, and for further laboratory bioassays, the LD50 value of 18 IJs/mL (approximately equal to 25 IJs/mL) was identified as the discriminating dosage. Maximum mortality was attained when bins were pre-wet for at least 1 min (>90% RH) and maintained at maximum humidity (>95% RH) post-treatment for at least 3 days (LT90=73 h), to ensure nematode survival and subsequent satisfactory infection of diapausing codling moth larvae. Tarping bins achieved the desired high level of humidity required. Furthermore, adjuvants (specifically Reverseal 10) also improved an application. The study conclusively illustrated that if all the above-mentioned conditions are met, H. zealandica has the potential to successfully disinfest wooden fruit bins of codling moth.
The potential of using an entomopathogenic nematode, Heterorhabditis zealandica Poinar, together with different test mulches (pine chips, wheat straw, pine wood shavings, blackwood and apple wood ...chips) to control diapausing codling moth, Cydia pomonella (L.) larvae was evaluated. Mesh cages were identified as a suitable larval-containment method. High levels of codling moth mortality were obtained when using pine wood shavings as mulch (88%) compared to pine chips, wheat straw, blackwood and apple wood chips (41-88%). Humidity (>95% RH) has to be maintained for at least 3 days to ensure nematode survival. It was also proven that nematodes had the ability to move out of infected soil into moist mulch, to infect the codling moth larvae residing at heights of up to 10 cm. Field experiments showed the importance of climatic conditions on nematode performance. Low temperatures (<15°C) recorded during the first trial resulted in low levels of control (48%), as opposed to the 67% mortality recorded during the second trial (temperatures ranged between 20 and 25°C). Low levels of persistence (<10%) were recorded in the mulches post-application. The study conclusively illustrated some of the baseline requirements fundamental to the success of entomopathogenic nematodes together with mulches for the control of codling moth.
The biocontrol potential of South African isolates of Heterorhabditis zealandica, Steinernema citrae, S. khoisanae, S. yirgalemense, and Steinernema sp., was evaluated against codling moth, Cydia ...pomonella. Codling moth was susceptible to all six nematode isolates at a concentration of 50 infective juveniles/insect (78-100% mortality). Low temperatures (10 h at 17°C; 14 h at 12°C) negatively affected larvicidal activity (less than or equal 3%) for all isolates. All tested isolates were most effective at higher levels of water activity (aw=1). The average aw50-values for all isolates tested was 0.94 (0.93-0.95), except S. khoisanae 0.97 (0.97-0.98). Regarding host-seeking ability, no positive attraction to host cues could be detected amongst isolates, except for H. zealandica. Three of the isolates, H. zealandica, S. khoisanae, and the undescribed Steinernema sp., were selected for field-testing and proven to be effective (mortality >50%). Insect containment methods used during field experimentation was shown to influence larvacidal activity, as different levels of mortality were obtained using various containment methods (wooden planks vs. pear tree logs vs. mesh cages). Pear tree logs were impractical. Predictive equations were subsequently developed, enabling future trials to be conducted using either planks or cages, enabling the prediction of the expected level of control on tree logs. All tested isolates therefore showed a certain degree of biological control potential, however, none of the experiments showed clear efficacy-differences amongst isolates. The study highlighted the importance of environmental factors to ensure the successful application of these nematodes for the control of diapausing codling moth larvae in temperate regions.
Abstract
During a survey for entomopathogenic nematodes in the Western Cape Province
of South Africa, a new species in the genus Heterorhabditis was collected
from a peach orchard. The nematode was ...trapped by the insect-baiting
technique using last instar larvae of Galleria mellonella. The infective
juvenile of the new species differs from the morphologically closest
species, H. marelatus, in shorter body length 600 (550-676) vs 685 (588-700)
μm and the shorter tail of 93 (86-108) vs 107 (99-117) μm. It differs from
all other species in the vulva pattern of hermaphroditic female. The genital
papillae of the male H. safricana n. sp. are typical for species in the
megidis-group (three papillae in terminal group). The average length of the
gubernaculum is longer than that of all other species (24 vs 19-23 μm) and
the gubernaculum length as a percentage of spicule length (53.9) is less
than that of H. mexicana (56), similar to that of H. floridensis (53.8), but
larger than that of all others (51 or less). For molecular characterisation,
the species closest to H. safricana n. sp. is H. marelatus. The length of
the ITS rDNA sequence of the new species is characterised by 995 base pairs,
identical to that of H. marelatus, but differs from this species by 25
aligned positions, seven of which are unambiguous autapomorphies.
Phylogenetic trees show further evidence of a separate species status for H.
safricana n. sp.
Adequate moisture levels are required for nematode survival and subsequent efficacy as entomopathogens. Formulation of nematodes aimed at aboveground applications may assist in maintaining such ...moisture levels. In this study, we report the effects of a superabsorbent polymer formulation, Zeba® on the performance of an entomopathogenic nematode, Heterorhabditis zealandica Poinar, for controlling diapausing codling moth, Cydia pomonella (L.) larvae in cryptic habitats on trees. Water activity (aw-value) on bark was considered to be an indication of moisture levels on trees in cryptic habitats where codling moth larvae are known to occur, thereby influencing nematode efficacy. H. zealandica was only able to infect codling moth larvae at aw≥0.92, with aw₅₀=0.94 and aw₉₀=0.96. Laboratory experiments in which nematode concentration was investigated indicated a positive linear relationship between the concentration of nematodes applied and the level of control obtained, with the highest level of mortality recorded at 80 IJs/larva, requiring at least 4 h of conditions conducive to nematode activity to ensure infectivity and subsequent efficacy. Further experimentation showed that the use of the Zeba formulation, together with the nematodes, improved the level of control obtained at 60% and 80% RH in the laboratory and that it also enhanced the survival and infection-ability of the nematodes in the field. The study conclusively illustrates that the tested formulation assisted in maintaining adequate moisture levels on the application substratum, as required for nematode survival and subsequent efficacy.
The biocontrol potential of South African isolates of Heterorhabditis zealandica, Steinernema citrae, S. khoisanae, S. yirgalemense, and Steinernema sp., was evaluated against codling moth, Cydia ...pomonella. Codling moth was susceptible to all six nematode isolates at a concentration of 50 infective juveniles/insect (78–100% mortality). Low temperatures (10 h at 17°C; 14 h at 12°C) negatively affected larvicidal activity (≤3%) for all isolates. All tested isolates were most effective at higher levels of water activity (aw=1). The average aw₅₀-values for all isolates tested was 0.94 (0.93–0.95), except S. khoisanae 0.97 (0.97–0.98). Regarding host-seeking ability, no positive attraction to host cues could be detected amongst isolates, except for H. zealandica. Three of the isolates, H. zealandica, S. khoisanae, and the undescribed Steinernema sp., were selected for field-testing and proven to be effective (mortality >50%). Insect containment methods used during field experimentation was shown to influence larvacidal activity, as different levels of mortality were obtained using various containment methods (wooden planks vs. pear tree logs vs. mesh cages). Pear tree logs were impractical. Predictive equations were subsequently developed, enabling future trials to be conducted using either planks or cages, enabling the prediction of the expected level of control on tree logs. All tested isolates therefore showed a certain degree of biological control potential, however, none of the experiments showed clear efficacy-differences amongst isolates. The study highlighted the importance of environmental factors to ensure the successful application of these nematodes for the control of diapausing codling moth larvae in temperate regions.
The potential of using an entomopathogenic nematode, Heterorhabditis zealandica Poinar, together with different test mulches (pine chips, wheat straw, pine wood shavings, blackwood and apple wood ...chips) to control diapausing codling moth, Cydia pomonella (L.) larvae was evaluated. Mesh cages were identified as a suitable larval-containment method. High levels of codling moth mortality were obtained when using pine wood shavings as mulch (88%) compared to pine chips, wheat straw, blackwood and apple wood chips (41–88%). Humidity (>95% RH) has to be maintained for at least 3 days to ensure nematode survival. It was also proven that nematodes had the ability to move out of infected soil into moist mulch, to infect the codling moth larvae residing at heights of up to 10 cm. Field experiments showed the importance of climatic conditions on nematode performance. Low temperatures (<15°C) recorded during the first trial resulted in low levels of control (48%), as opposed to the 67% mortality recorded during the second trial (temperatures ranged between 20 and 25°C). Low levels of persistence (<10%) were recorded in the mulches post-application. The study conclusively illustrated some of the baseline requirements fundamental to the success of entomopathogenic nematodes together with mulches for the control of codling moth.
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