Conservation biological control is the modification of the environment or existing practices to protect and enhance antagonistic organisms to reduce damage from pests. This approach to biological control has received insufficient attention com- pared with inundative applications of microbial antagonists to control nematodes. This review provides examples of how production practices can enhance or diminish biological control of plant-parasitic nematodes and other soilborne pests. Antagonists of nem- atodes can be enhanced by providing supplementary food sources such as occurs when organic amendments are applied to soil. However, some organic amendments (e.g., manures and plants containing allelopathic compounds) can also be detrimental to nematode antagonists. Plant species and genotype can strongly influence the outcome of biological control. For instance, the susceptibility of the plant to the nematode can determine the effectiveness of control; good hosts will require greater levels of suppression than poor hosts. Plant genotype can also influence the degree of rhizosphere colonization and antibiotic production by antagonists, as well the expression of induced resistance by plants. Production practices such as crop rotation, fallow periods, tillage, and pesticide applications can directly disrupt populations of antagonistic organisms. These practices can also indirectly affect antagonists by reducing their primary nematode host. One of the challenges of conservation biological control is that practices intended to protect or enhance suppression of nematodes may not be effective in all field sites because they are dependent on indigenous antagonists. Ultimately, indicators will need to be identified, such as the presence of particular antagonists, which can guide decisions on where it is practical to use conservation biological control. Antagonists can also be applied to field sites in conjunction with conservation practices to improve the consistency, efficacy, and duration of biological control. In future research, greater use should be made of bioassays that measure nematode suppression because changes in abundance of particular antagonists may not affect biological control of plant parasites.

Conserving and Enhancing Biological Control of Nematodes.

The endospore-forming bacterium Pasteuria penetrans is an obligate parasite of root-knot nematodes (Meloidogyne spp.). The primary objective of this study was to determine the effect of crop sequence on abundance of P. penetrans. The experiment was conducted from 2000 to 2008 at a field site naturally infested with both the bacterium and its host Meloidogyne arenaria and included the following crop sequences: continuous peanut (Arachis hypogaea) (P-P-P) and peanut rotated with either 2 years of corn (Zea mays) (C-C-P), 1 year each of cotton (Gossypium hirsutum) and corn (Ct-C-P), or 1 year each of corn and a vegetable (V-C-P). The vegetable was a double crop of sweet corn and eggplant (Solanum melongena). A bioassay with second-stage juveniles (J2) of M. arenaria from a greenhouse (GH) population was used to estimate endospore abundance under the different crop sequences. A greater numerical increase in endospore densities was expected in the P-P-P and V-C-P sequences than in the other sequences because both peanut and eggplant are good hosts for M. arenaria. However, endospore densities, as determined by bioassay, did not substantially increase in any of the sequences during the 9-year experiment. To determine whether the nematode population had developed resistance to the resident P. penetrans, five single egg-mass (SEM) lines from the field population of M. arenaria were tested alongside the GH population for acquisition of endospores from the field soil. Four of the five SEM lines acquired 9 to 14 spores/J2 whereas the GH population and one of the SEM lines acquired 3.5 and 1.8 spores/J2, respectively. Endospore densities estimated with the four receptive SEM lines were highest in the P-P-P plots (14-20 spores/J2), intermediate in the V-C-P plots (6-7 spores/J2), and lowest in the Ct-C-P plots (< 1 spore/J2). These results indicate that the field population of M. arenaria is heterogeneous for attachment of P. penetrans endospores. Moreover, spore densities increased under intensive cropping of hosts for M. arenaria, but the GH population of the nematode was not receptive to spore attachment. However, previously, the GH population was very receptive to spore acquisition from this field site. One explanation for this inconsistency is that the M. arenaria population in the field became resistant to the dominant subpopulation of P. penetrans that had been present, and this led to the selection of a different subpopulation of the bacterium that is incompatible with the GH population.

Population Dynamics of Meloidogyne arenaria and Pasteuria penetrans in a Long-Term Crop Rotation Study

Immunological quantification of the nematode parasitic bacterium Pasteuria penetrans in soil

The effect of nematode population density at the time of application and formulations of in vitro-produced Pasteuria spp. endospores on the final population density of Belonolaimus longicaudatus was studied in an 84-d-long pot bioassay. The experiment utilized a factorial design consisting of 30 or 300 B. longicaudatus /100 cm3 of sandy soil and three formulations of in vitro-produced Pasteuria spp. endospores (nontreated, granular, or liquid). No differences were observed in percent endospore attachment between nematode inoculum levels during either trial. Granular and liquid formulations of in vitro-produced endospores suppressed nematode population densities by 22% and 59% in the first trial and 20% and 63% in the second, respectively compared with the nontreated control. The liquid formulation increased percent endospore attachment by 147% and 158%, respectively, compared with the granular formulation. The greatest root retention by the host plant was observed at the lower B. longicaudatus inoculation level following application of the liquid formulation. While both the granular and liquid formulations reduced B. longicaudatus population densities in the soil, the liquid spore suspension was most effective.

Effects of formulation and host nematode density on the ability of in vitro-produced pasteuria endospores to control its host Belonolaimus longicaudatus.

A population of the citrus nematode Tylenchulus semipenetrans Cobb (Tylenchida: Tylenchulidae) associated to a Pasteuria sp. (Bacillales: Bacillaceae) was studied in a naturally infested field. In a first population dynamics study, prevalence never exceed 50 % and showed a density-dependent relationship with the host at 2–3 months time lags, with a spring sharp increase. In a spatial sampling study, both organisms resulted uniformly distributed and matched the observed relationship. In top (10 cm) soil the adult females were lower than in deeper layers, whereas the nematodes with adhering endospores were higher. Anderson and May’s Model G applied showed that 50–400 endospores per cc of soil can sustain stable regulatory cycles. The nematode and bacterium populations were found in the field 12 years later with declining densities, below the calculated threshold needed to maintain the parasite infection. Data confirmed the bacterium potential in nematode regulation and showed that host numbers affect prevalence changes in time.

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Erratum to: Regulation of the citrus nematode Tylenchulus semipenetrans by a Pasteuria sp. endoparasite in a naturally infested soil

The population dynamics of Meloidogyne incognita juveniles (J2) in association with the parasite Pasteuria penetrans was studied in a field in Martinique. Data obtained at three-week intervals for one year were analyzed to study the relationship between J2 density and endospore infestation levels. Six crop sequences were evaluated in five replicates. They were: resistant tomato, okra, fallow, cabbage; susceptible tomato, okra, fallow, cabbage; bean, pepper, fallow, cabbage; eggplant, fallow; cucumber, velvetbean, fallow, cabbage; fallow, pepper, cabbage, fallow, cabbage. The J2 densities showed fluctuating trends, frequently synchronous with fluctuations of endospore infestation. The highest J2 numbers observed ranged between 1 000 and 1 500 nematodes/100 g of soil in the first four crop sequences, and between 150 and 250 J2/100 g of soil in the last two. The highest percentages of nematodes with adhering endospores ranged between 50 and 70%. The resistant tomato crop showed a sharp decrease in the nematode density at the end of the crop sequence, with a similar synchronous decrease of the endospore infestation. This trend was also observed during the fallow periods in the other treatments. Susceptible crops allowed a sharp increase of the J2 densities. During these periods of population growth, the concomitant increase of the endospore infestation levels was interrupted with fast growing J2 densities. In the last two crop sequences, the J2 densities progressively decreased because of the on-host status of velvetbean and of a longer fallow, respectively. When all the pooled data from the six treatments were considered, a significant linear correlation was observed between J2 densities and the corresponding endospore infestation values (P 0.001). This relationship appeared related to the density of females releasing J2 and/or endospores in the soil. All pooled mean densities also followed the Gutenberg-Richter power law distribution, indicating the oc-currence of chaotic effects influencing their dynamics. Although chaos implies non-predictability of J2 numbers and endospore infestation dynamics, it also suggests that large-scale fluctuations may induce local nematode suppression by P. penetrans.

Aurelio Ciancio

Papers

Population dynamics of Meloidogyne incognita and infestation levels by Pasteuria penetrans in a naturally infested field in Martinique.