Mark O. Harrell

Influences of Trees on Abundance of Natural Enemies of Insect Pests: A Review

In this article we review the use of natural enemies in crop pest management and describe research needed to better meet information needs for practical applications. Endemic natural enemies (predators and parasites) offer a potential but understudied approach to controlling insect pests in agricultural systems. With the current high interest in environmental stewardship, such an approach has special appeal as a method to reduce the need for pesticides while maintaining agricultural profitability. Habitat for sustaining populations of natural enemies occurs primarily at field edges where crops and edge vegetation meet. Conservation and enhancement of natural enemies might include manipulation of plant species and plant arrangement, particularly at these edges; and consideration of optimum field sizes, number of edges, and management practices in and near edges. Blending the benefits of agricultural and forestry (windbreak) systems is one promising approach to field edge management that has additional benefits of wind protection and conservation of desirable wildlife species.

Worldwide invasion routes of the pinewood nematode: What can we infer from population genetics analyses?

Identifying the invasion routes and determining the origin of new outbreaks of invasive species are of crucial importance if we are to understand the invasion process, improve or establish regulatory measures and, potentially, limit the damage. We focused here on the invasion of Europe by the pinewood nematode (PWN), Bursaphelenchus xylophilus (Steiner & Buhrer, 1934; Nickle 1970; Nematoda: Aphelenchoididae), a major pest of forest ecosystems, native to North America and already invasive in Asia since the beginning of the twentieth century. We evaluated the genetic diversity and structure of worldwide field PWN samples by classical and Bayesian population genetics methods to determine the source of the European invasive populations and the number of introduction events in Europe. We found (1) a very strong spatial genetic structure in native PWN populations, (2) a very low level of polymorphism in each of the invaded areas and (3) contrasted results concerning the origin of European invasive populations. Our findings provide evidence for: (1) a large effect of genetic drift on the biological cycle of the PWN, due to intense demographic bottlenecks during tree infections, not compensated for by effective dispersal of its vector; (2) a single introduction event for each of the invaded areas in Japan and Europe and a small effective size for the introduced populations and (3) a mainland Portuguese origin for PWN populations from Madeira. However, more sophisticated methods of invasion route inference and broader sampling are required to conclusively determine the origin of the European outbreak.

Assessment of Microenvironmental Conditions Related to the Use of Synthetic Sheet Mulches for Protecting Newly Planted Trees in Semi-Arid Environments

ABSTRACT Microenvironmental conditions created by six weed control methods and the resulting survival and height growth of two species of windbreak trees were measured in three separate plantings in Nebraska. The treatments included three synthetic sheet mulches commonly used in tree plantings. Climagro, a black polyethylene mulch, and Sunbelt, a black woven polypropylene mulch, improved the growth of green ash (Fraxinus pennsylvanica Marsh.) in two of the three plantings. Survival of green ash and survival and growth of Scotch pine (Pinus sylvestris L.) were not significantly different among the treatments. Positive correlations were found between August soil moisture and ash height growth. No relationship was found between soil temperature and ash growth, although significant differences in soil temperature among treatments were detected. Weed control treatment did not affect air temperature above the soil surface or the amount of mycorrhizal roots on Scotch pine. Economic analyses of the use of Climagro and Sunbelt mulches in a hypothetical green ash windbreak investment resulted in positive returns.