Browsing by Author "Dlamini, Thandwa"

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    Prospects for using Entomopathogenic Nematodes as a biocontrol agent against Western flower thrips Frankliniella occidentalis (Thysanoptera: Thripidae)
    (Stellenbosch : Stellenbosch University, 2018-12) Dlamini, Thandwa; Malan, Antoinette P.; Allsopp, Elleunorah; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thripidae: Thysanoptera), is one of the most economically important pests in greenhouses, with preference being exhibited towards feeding on flowers. WFT is a serious pest of greenhouse cultivation, because it damages plants directly by means of feeding and oviposition on foliage and flowers, and indirectly, by means of vectoring tospoviruses, such as impatiens necrotic spot virus and tomato spotted wilt virus. Approximately 7500 species of thrips have been identified to date, with 14 species being recognised as virus vectors, of which F. occidentalis is responsible for transmitting five species of tospoviruses. Chemical control has been the most frequently used method for the control of WFT in greenhouses. The high frequency of insecticide applications for WFT control, coupled with the short generation time of F. occidentalis, has led to an increasing incidence of insecticide resistance in WFT in recent years. An integrated pest management (IPM) programme offers a sustainable alternative control for WFT in undercover production. Biological control, especially the use of entomopathogenic nematodes (EPNs), has been identified as an environmentally friendly option. The use of other parasites and predators for biological control has shown only limited ability to reduce WFT populations, apparently because their movement is restricted when entering tight flower buds, meristem tissues, or narrow flower structures favoured by WFT, due to their large body size. This study investigated the potential use of indigenous EPNs for the control of WFT under laboratory and greenhouse conditions. To achieve the above, the development and survival rate of F. occidentalis on two host plants, as well as its biology, were studied under laboratory conditions to identify life stages targetable with EPNs. The efficacy of the local strains of EPNs to control the different life stages of WFT, and the optimum nematode concentrations required for the suppression of WFT under laboratory conditions, were investigated. Lastly, the potential of foliar and soil applications of different concentrations of locally isolated S. yirgalemense for controlling F. occidentalis in a commercial blueberry greenhouse was investigated. Laboratory studies were conducted to determine the life-history and host preference of adult WFT on chrysanthemum (Dendranthema grandiflora) leaflets and green bean pods (Phaseolus vulgaris). The identification of Frankliniella occidentalis was verified, using both morphological and molecular methods. Main morphological features included six to nine antennal segments, major setae on the head and pronotum dark, interocellar and postocular setae approximately the same length, the first vein of the anterior wing with a complete row of regularly spaced setae, and posteromarginal comb on tergite VIII of the female well-developed and complete. Molecular identification was based on amplification of the mtCOI gene sequences for the identification of four thrips species (F. occidentalis, Thysanoptera sp., Gynaikothrips ficorum and Pseudophilothrips ichini) collected from the study area. The F. occidentalis morphologically identified showed 100 % identity with sequences in the database from GenBank. One of the Thrips sp. could not be identified neither morphologically nor molecularly and could possibly be an unidentified species. Results from the life-history study showed that more first instar larva hatched on chrysanthemums, faster larval developmental rate and a higher survival rate on chrysanthemums indicating that chrysanthemum is a more attractive and more suitable host than green bean. Among the 12 EPN species tested against F. occidentalis in laboratory bioassays, virulence ranged from 11 % to 67 %. Generally, Heterorhabditis spp. were more virulent than the Steinernema spp. Heterorhabditis baujardi was found to be the most potent species, with a mortality of 67 %, although it was not significantly different from Steinernema yirgalemense (66 %). The study showed that the commercial nematode Steinernema feltiae did not perform better than the local EPN species. Bioassays to determine infectivity were performed using different life stages (larva, pupa and adult) of F. occidentalis exposed to infective juveniles (IJs) of S. yirgalemense, H. baujardi and Steinernema jeffreyense. The pupae of WFT were found to be more sensitive to nematode infection than either the larvae or the adults. The highest WFT mortality was recorded for the pupae (72 %) when applying 100 IJs/insect of H. baujardi, with the lowest being recorded when treated with S. jeffreyense (17 %). Steinernema yirgalemense and H. baujardi were tested at concentrations of 0, 10, 20, 40, 80, and 160 IJs/larva. Increasing EPN concentrations gave increased thrips mortality, with a probit analysis indicating S. yirgalemense to be 5.49 more potent than H. baujardi. Results from the temporal development study showed that both S. yirgalemense and H. baujardi were able to complete their life cycles in the host within 5 days, and were able to produce a new cohort of IJs. Relatively few IJs were found to penetrate the insect, due to the small size of the insect and the IJs recovered from the host were relative in number to the IJs penetrated. The field trial was initiated to determine the efficiency of different concentrations of S. yirgalemense in controlling F. occidentalis in a commercial blueberry greenhouse. A combination of foliar and soil applications of S. yirgalemense in two greenhouse trials, one at lower concentrations of 4.3, 8.6, and 17.2 IJs/cm2, and the other at higher concentrations of 25, 50, and 100 IJs/cm2 were applied. The results in both trials indicated thrips mortality < 50 % at the highest concentration of 100 IJs/cm2, at mean substrate temperatures < 15 °C, which was below optimum for S. yirgalemense infection. Increase in nematode concentration resulted in a decline in the number of thrips captured. The experiment with higher concentrations showed increased thrips mortality (53 %) in relation to the experiment with lower concentration (< 40 %). Steinernema yirgalemense was persistent for 4 weeks, with low mortalities when mealworms were used to monitor infectivity. The correct identification of thrips is important for further studies investigating biological control thereof. Research into the use of EPNs for the biological control of insects should not be restricted to laboratory conditions, as these conditions do not truly represent field performance. Steinernema yirgalemense showed potential for use as a biocontrol option for WFT, giving low to moderate results in the field trial, under suboptimal temperatures, at a concentration of 100 IJs/cm2. The application of S. yirgalemense to control WFT requires further investigation under relatively warmer substrate temperatures in the Haygrove tunnels under blueberry production. Application of nematodes should target WFT populations on new growth after post-harvest pruning, when WFT causes significant economic damage. Weekly follow-up applications should be investigated as a future alternative. The feasibility of applying S. yirgalemense in conjunction with other biological agents and insecticide–pathogen synergistic interactions in IPM systems should also be investigated.