Abiotic and biotic factors influencing host-plant use of a generalist herbivore through plant-mediated interactions: oviposition and larval performance by the bertha armyworm, Mamestra configurata Walker (Lepidoptera: Noctuidae) on canola

  • Author / Creator
    Chaminda De Silva Weeraddana
  • The bertha armyworm (BAW), Mamestra configurata Walker (Lepidoptera: Noctuidae), is a native, polyphagous herbivore which feeds on a variety of plants in different families. In the Prairie Provinces, canola, Brassica napus L. (Brassicaceae), is a preferred host, making BAW a significant pest. Canola is impacted by a variety of abiotic and biotic stressors that could directly compete with BAW or indirectly influence the performance of BAW through plant-mediated interactions. In this study, I tested for plant-mediated interactions of canola plants that were exposed to three different stressors: 1. various levels of nutrition; 2. oviposition and herbivory by a specialist herbivore; and 3. root pathogen infection; on subsequent host use by BAW. Crop fertilization may have an effect on efficacy of pest control in Integrated Pest Management (IPM) programs. Fertilization of crop plants and the use of resistant canola varieties may directly influence the performance of BAW larval feeding and development. I altered the fertilization level on three different canola varieties: an early flowering variety: ‘5535 CL’; a late flowering variety: ‘6060RR’; and a control variety: ‘Q2’ for experiments. Bertha armyworm females laid more eggs and larvae performed better on plants treated with moderate and high versus low fertilizer treatments. Plant leaf tissue nutrient content (nitrogen, phosphorous, potassium) and plant growth were also higher in plants that received moderate and high levels of fertilization. The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae), is a specialist herbivore on plants in the Brassicaceae and one of the most significant pests of canola in western Canada. Diamondback moth co-occurs with BAW in canola fields but DBM herbivory occurs prior to BAW herbivory. In this study, I tested the effect of DBM oviposition and herbivory on subsequent host use by BAW. Bertha armyworm larvae fed more on plants with DBM eggs. Bertha armyworm adults laid fewer eggs when exposed to canola plants with DBM eggs. Plant hormone analyses suggested that DBM oviposition suppresses canola defenses. I also subjected DBM-infested, mechanically damaged and uninfested canola plants to subsequent BAW herbivory. Bertha armyworm larvae were negatively influenced by DBM feeding damage whereas BAW oviposition was not affected. Taken together, all these results suggest that herbivory by the specialist, DBM, negatively influences subsequent host use and larval performance by the generalist, BAW on canola. Clubroot disease, caused by a soil borne protist, Plasmodiophora brassicae Woronin, has recently emerged as an important agricultural pest that impacts canola production and yield. Both P. brassicae and BAW infestation occur in the agroecosystems of Alberta canola fields. Thus, it is important to study the potential interaction between P. brassicae diseased plants and BAW to properly manage both threats. The effect of P. brassicae disease infection on subsequent BAW oviposition and larval development was tested using two hybrid canola varieties that varied in resistance to the pathogen (‘45H26’ [susceptible to P. brassicae ] and ‘45H29’ [resistant to P. brassicae ]). Adult female moths preferentially layed eggs on the susceptible canola in the absence of P. brassicae inoculation. Inoculation of resistant canola with P. brassicae did not influence oviposition of BAW. Inoculation with P. brassicae influences larval development as larvae weigh more when reared on non-inoculated as compared with inoculated resistant canola. In contrast, heavier larvae resulted from feeding on inoculated as compared with non-inoculated susceptible canola. Plant hormone analyses suggest that canola defenses are induced in response to P. brassicae inoculation. Large amounts of salicylic acid (SA) conjugates are present in the inoculated susceptible canola that may influence BAW oviposition. In addition, inoculation with P. brassicae changes the volatile profile of chemicals released by canola plants which likely mediate oviposition responses by BAW females. These results show that interactions between BAW and canola are affected by host plant nutrition, pathogen infection and previous damage by a specialist herbivore. The importance of canola-mediated responses to abiotic and biotic stressors on subsequent host use by the generalist herbivore, BAW, improves our understanding of canola agroecosystems and provides a foundation for the improvement of IPM of canola in Alberta.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
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