Biological control of sorghhm and rice stem borers, chilo partellus and sessamia calamistis using endophytic strains of beauveria bassiana.
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Date
2018
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Abstract
Sorghum and rice are two of the major cereals grown across the world. Both of these crops are subjected to a range of abiotic and biotic constraints. Insect pests are important biotic stress factors, which affect both of the crops at all of their growth stages. Stem borers from the family of Lepidoptera, e.g. Chilo partellus (Lepidoptera: Pyralidae) and Sesamia calamistis Hampson (Lepidoptera: Noctuidae) are important pests that attack these cereals. Control of C. partellus and S. calamistis has largely been with pesticides. However, chemical pesticides are too expensive for most small-scale farmers in Africa, leaving their crops unprotected. Biological control is one of the measures that have been advocated for the management of stem borers. Various strains of Beauveria bassiana (Vuillemin) have been documented as being endophytes infecting a wide range of plants, as well as being pathogenic on numerous insect pests. Successes in biological control research have led to the development of various B. bassiana products, which are available commercially, but these are largely epiphytic strains. Biological control studies were therefore conducted with several endophytic strains of B. bassiana against sorghum stem borer, C. partellus, and the rice stem borer, S. calamistis. The fungi were tested by endophytic behaviour and the ability to control the 3rd larval instars of both stem borers, in the laboratory and greenhouse. The interactions of B. bassiana strains and a commercially available Trichoderma product, Eco-T®, were tested in sorghum and rice plants.
In vivo and in vitro screening were initially undertaken to evaluate the endophytic behaviour of 20 B. bassiana strains, using two inoculation methods. Subsequently, the best endophytic B. bassiana strains and the best inoculation method were tested at 30 and 60 days after inoculation. The strains were screened in vivo using seed treatments and foliar sprays, under greenhouse conditions, for endophytic behaviour in sorghum and rice plants. There were highly significant differences between the B. bassiana strains (P = 0.0001). Depending upon the inoculation method, the B. bassiana strains that successfully colonized the sorghum and rice plants could be selected after 30 and 60 days. Five strains of B. bassiana strains (Bb3, Bb4, Bb10, Bb21 and Bb35) were found to be endophytic in both crops, and to provide biological control against the two borers.
The best five B. bassiana strains were tested for their pathogenicity on the 3rd instar larvae of C. partellus and S. calamistis. Out of the five endophytic strains of B. bassiana, two (Bb35 and Bb3) were the most pathogenic on C. partellus, with the greatest mortality of 80 % being achieved within 28 days after treatment. The B. bassiana strains Bb35 and Bb4 were the most effective strains against S. calamistis, killing 93.33 and 76.66% of the 3rd larval instar at 28 days, respectively. The cumulative mortality of the 3rd instar larvae of both stem borers increased over time at 21 days after inoculation for all five B. bassiana strains.
A field trial was conducted to evaluate the biocontrol efficacy of the five best endophytic strains of B. bassiana against C. partellus, compared to pyrethroid pesticide, Karate. Three of the endophytic strains of B. bassiana strains were as effective as Karate sprays when they were applied as seed treatments, reducing damage by C. partellus as much as Karate did.
In vitro and in vivo screening were carried out under laboratory and greenhouse conditions, using various inoculation methods, to assess the interaction between the five B. bassiana strains and a commercially available Trichoderma harzianum product, Eco-T®. In the in vitro dual culture bioassay, one of the five endophytic B. bassiana strains (Strain Bb35) was not inhibited by T. harzianum Strain Kd (TKD) at 15 days after inoculation at 30 days after in vitro inoculation. None of the five endophytic B. bassiana strains grew in the presence of TKD. Only the TKD grew all over the plates. In greenhouse trials, various interactions occurred between the two fungi, according to the inoculation methods. When a mixture of conidia of the two fungi was used at the same time as a seed treatment, there was a strong inhibitory effect by TKD toward the five B. bassiana strains. However, if sorghum plants were seed treated with the five B. bassiana strains, followed by drenching of the plant roots with a TKD suspension, then the B. bassiana strains appeared to be able to colonize the stems of the plants whilst the TKD colonized the roots. Sorghum roots were rapidly colonized by the TKD when it was used alone for the seed treatment.
The endophytic behaviour of some strains of B. bassiana in sorghum and rice plants can be used as powerful tool to enhance their biological control activity against stem borers of these crops. However, the tested TKD and B. bassiana strains were not compatible in the same space, such as the rhizosphere, but could be used sequentially to secure the benefits of insect control by the B. bassiana strains, as well as the biological control and plant growth stimulation activities provided by T. harzianum strains.
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Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.