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Insect Pathogens as Endophytes – New Tools for Pest Control?

Final Report Summary - ENDOPEST (Insect Pathogens as Endophytes – New Tools for Pest Control?)

The goal of this project was to establish the potential of endophytic entomopathogenic fungi as biological plant protection agents using plant defense theory as a framework, B. bassiana as a model fungus and wheat as a model plant. More specifically the original objectives were:

1. For the first time to establish the effects of endophytic B. bassiana on physiology and reproduction of vertebrate pests and reproduction of invertebrate pests using the house mouse and the bird cherry oat aphid as model pests.
2. For the first time to establish whether these herbivores will favor or avoid the infected plants over endophyte-free plants.

No studies considering the B. bassiana-plant interactions in a context dependent framework such as resource availability have been published. Therefore experiments were considered within a context of nutrient availability.

Description of the work performed

Establishment of the model system
In the initial experimental part of the project, the model system was established by inoculating B. bassiana to wheat, tomato, maize and Arabidopsis. Maize plants treated with B. bassiana were observed growing significantly faster and taller than control plants. Therefore, maize was chosen as the model plant in the experiments. During this process, new contacts were established with a group in New Zealand on critical approaches to the inoculation techniques published, which eventually led to a joint review paper on methodology of using B. bassiana as an endophyte.

Seed treatment was used to inoculate plants. Colonization success and frequency of establishment of B. bassiana was evaluated by application of advanced polymerase chain reaction (PCR) methods with Beauveria–specific primers. Plants treated with B. bassiana were grown with untreated control plants in greenhouse facilities until the beginning of the experiments.

Hypothesis I: B. bassiana seed treatment improves the growth of maize only in high nutrient conditions

A greenhouse experiment evaluating the effects of B. bassiana seed treatment on maize growth was established. The first trial of the experiment included 7 replicates and the second trial included 4 replicates (randomized complete block design). The growth of the plants was monitored both in high (mimicking agricultural field conditions) and low nutrient conditions (representing poor soil). The height and developmental stages of the plants was monitored every three days for 6 weeks until the end of the experiment. Then the plants were harvested and the above and below ground biomasses of the plants were measured.

Hypothesis II: B. bassiana seed treatment negatively affects population growth of the bird cherry oat aphid.

The effects of B. bassiana seed treatment of maize on population sizes of bird cherry oat aphids were tested in one greenhouse experiment with two trials. In each of the two trials, 36 maize plants growing in pots (18 endophyte–infected and 18 endophyte–free plants) were placed individually in sealed plastic containers in a randomized complete block design. Five adult aphids were introduced onto each of experimental plants. The number of aphids was counted every day for 10 days.

Hypothesis III: Aphids prefer untreated maize over maize treated with B. bassiana.

To test the effects of B. bassiana seed treatment on aphid host plant preference, an aphid choice experiment was conducted with two trials. For each of the two experimental trials (n = 18 for each pairwise comparison in each of the experiments), a piece of a fresh maize leaf from both B. bassiana treated and control plants was placed symmetrically on a petri dish at room temperature. One naïve aphid that had been reared on oats was placed centrally in each petri dish and allowed to choose between the two leaves. The choice of each aphid was recorded at 1, 5 and 24 h. If aphids did not choose between either of the two leaves, the preference was recorded as 0.

Main results achieved

B. bassiana seed treatment significantly increased the height, shoot and root weight, root:shoot ratio and speeded up the development of the maize plants compared to controls, but only in high nutrient conditions when there were no aphid herbivores on the experimental plants. When aphids were feeding on the plants, plant growth was reduced even when nutrients were abundantly available. B. bassiana treatment significantly reduced the population growth of aphids. However, the aphids equally often chose the control plants over the B. bassiana treated plants in the experiments. These results suggest that B. bassiana seed treatment may induce defences against aphid attack leading to lowering of their population growth but at a cost of plant growth. Fungus treatment did not influence aphid host plant selection.

The impact and use of the results

The findings are the first to report that 1) the growth promotion effect of B. bassiana is context dependent and relies on nutrient availability and presence of herbivores, and 2) the effects of B. bassiana seed treatment on pest resistance and host plant performance can be contradictory. The plant-fungus relationship is therefore complex and context dependent. These results suggest that B. bassiana seed treatment could be used to boost plant growth of maize in agricultural management but only when nutrients are abundantly available and pest herbivores are effectively eliminated. Presence of B. bassiana may represent a resource sink when nutrients are limited or when aphids are present. The results indicate that B. bassiana seed treatment may be applied as a new tool for biological pest control and plant growth promoter when nutrients are abundantly available in the soil, while the fungus does not provide any growth benefits when nutrients are scarce. The findings emphasize the importance of considering resource availability when evaluating the beneficial effects of inoculating plant with entomopathogenic fungi.