Host plant resistance is often viewed as being complementary to biological control as a pest control approach, but some plant characteristics that confer resistance to herbivores could potentially reduce the effectiveness of their natural enemies. New cultivars of alfalfa, Medicago sativa, have been released with glandular trichomes for resistance to potato leafhopper, Empoasca fabae. These trichomes, hair-like appendages extending from the leaves and stems, can interfere with herbivore movement, feeding and oviposition, may secrete chemicals that aid in plant defense, or may be smell or taste repellents. Yet, the impact of these glandular trichomes on the primary natural enemy of the leafhopper, the egg parasitoid Anagrus nigriventris, is unknown.
The host searching behavior of A. nigriventris was compared on four alfalfa clones varying in trichome characters: Ranger, a susceptible clone with relatively sparse trichomes; B14, a resistant clone with dense but nonglandular trichomes; and FG12 and FG18, two resistant clones with glandular trichomes.
The wasps spent equal amounts of time drumming, probing, and grooming on the four clones. However, they were less likely to forage and spent less time searching for leafhopper eggs on the two varieties with glandular trichomes than on the other two varieties, and they made significantly fewer ovipositional probes on these plants. They tended to fly off of clones with glandular trichomes more often than off of clones with nonglandular trichomes.
Grandular trichomes interfere with A. nigriventris' searching behavior, which may seriously reduce the effectiveness of A. nigriventris on leafhopper egg parasitism. However, these studies were done in the laboratory and the effects may not be as significant in the field, so field testing should be done to determine the degree of glandular trichome interference with parasitism by A. nigriventris. If such interference does occur under field conditions, future breeding research should aim toward finding resistance that does not hinder this important natural enemy.
Lovinger, A., D. Liewehr, and W. O. Lamp. 2000. Glandular trichomes on alfalfa impede searching behavior of the potato leafhopper parasitoid. Biol. Control 18(3):187-192.
Root Colonizing Bacteria and Disease Suppression
There are several traits that may affect the ability of bacterial biocontrol agents to colonize and protect plants from disease pathogens. Chemotaxis (orientation toward a chemical produced by plant roots), motility, and sporulation are all assumed to be significant in disease suppression, but different research projects have resulted in contradictory reports. Antagonism, or the production of an antibiotic, is another important factor.
Bacillus megaterium strain B153-2-2 is a potential bacterial biocontrol agent against Rhizoctonia solani. Originally isolated from the crown of a field-grown soybean plant, it colonized soybean roots with high populations and showed significant positive chemotactic response to soybean roots. The roles of the four traits in disease suppression were tested by comparing seven mutants of the bacterium (with altered antagonism, chemotaxis, motility, and/or sporulation) during seed and root colonization in artificial soil mixes in the greenhouse. The bacterial cells were introduced into "coarse" or "fine" soil separately either as a soybean seed coating or soil application.
The bacteria colonized soybean roots better in the coarse soil mix, especially the mutants with increased chemotaxis and motility, and disease was reduced. In both soil mixes, the pathogen tended to reduce root colonization early, probably because of competition for space or nutrients, but differences in root colonization diminished as seedlings grew older.
Bacterial antagonsim appeared to be an important factor in disease suppression, since the one antagonism defective mutant showed a significant increase in disease severity. The sporulation-defective mutants had low cell populations immediately after application and, therefore, lower root colonization than the others.
Those strains that colonized the roots best also were the ones that provided the best suppression of Rhizoctonia root rot. Root colonization by a biocontrol agent is subjected to dynamic equilibrium of a complex microorganism community, and success in colonization may vary depending on timing and numerous factors in the soil environment. Therefore, results with biocontrol agents may vary from application to application.
Zheng, X. Y. and J. B. Sinclair. 2000. The effects of traits of Bacillus megaterium on seed and root colonization and their correlation with the suppression of Rhizoctonia root rot of soybean. Biocontrol 45(2):223-243.
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