Biological Control Efficacy Research Articles

Predators impairing the natural biological control of parasitoids

A well known case of ineffective natural biological control: the puzzling coexistence of the coffee leaf miner, Leucoptera coffeellum (Guérin-Mèneville), and its natural enemies was analyzed. Despite being a suitable prey to eight parasitoid species and three wasp species, all occurring simultaneously, the coffee leaf miner too often presents populations far above the damaging level for the coffee plantation. It is demonstrated that predatory wasps and parasitoids interact negatively, possibly because predatory wasps kill parasitized miner's larvae. In doing so, predatory wasps indirectly kill parasitoids, thereby impairing the efficacy of the natural biological control. It is warned that biological control programs should be based on knowledge of food web interactions, rather than simply on strategies involving introduction of exotic natural enemies.

Spatial Simulation of Epizootics Caused by Beauveria bassiana in Russian Wheat Aphid Populations

Models of disease dynamics commonly make the assumption of spatial homogeneity in the underlying host population. However, insect behavior may result in spatially heterogeneous populations with which pathogens interact. We modified a simulation model of temporal and spatial population dynamics of the Russian wheat aphid, Diuraphis noxia, on preferred or nonpreferred host plants, by incorporating effects of the entomopathogenic fungus, Beauveria bassiana. Epizootic parameters included time from inoculation of aphids until death, duration of sporulation, and estimated exposure probability. Simulations first predicted results of previously described experiments in which D. noxia adults were inoculated with conidial suspensions or water and placed on wheat or oat seedlings in 81-plant grids in cages. Subsequently, large-scale simulations were run for hypothetical field situations on 50 × 50-plant grids of wheat or oat. With B. bassiana present for both cage and larger scale simulations, results indicated that, on oat, an expanding infection front lagged behind the expanding aphid population front. Continual aphid movement from hosts resulted in many escapes, and the aphid population persisted at slightly reduced levels. On the preferred wheat host, patterns developed with pockets of infected aphids and other pockets of healthy aphids. Localized aphid populations that escaped initial infestation were able to proliferate, whereas other local populations were greatly reduced or became extinct due to lack of movement from the hosts, resulting in increased exposure to pathogen inoculum. Thus, proliferation and fluctuation of the pathogen were strongly influenced by the plant hosts' effects on aphid movement behavior. Incorporating spatial dynamics into disease models should prove useful in other efforts to predict biological control efficacy by entomopathogenic fungi in heterogeneous habitats.

Construction of a range of derivatives of the biological control strain agrobacterium rhizogenes K84: a study of factors involved in biological control of crown gall disease

The biological control strain Agrobacterium rhizogenes K84 is an effective agent in the control of Agrobacterium pathogens, the causative agents of crown gall disease. A number of factors are thought to play a role in the control process, including production of the specific agrocins 84 and 434, which differ in the spectra of pathogenic strains that they inhibit in vitro. A range of derivatives of strain K84 has been developed with every combination of the three resident plasmids, pAgK84, pAgK434, and pAtK84b, including a plasmid-free strain. These derivatives produced either both, one, or neither of the characterized agrocins 84 and 434 and were isolated by plasmid curing, conjugation, and Tn5 transposon mutagenesis. The ability of the derivative strains to inhibit gall formation on almond roots was compared to that of the wild-type K84 parent. Treatment with the plasmid-free derivative did not result in a significant level of control of an A. rhizogenes pathogen based on numbers or dry weight of galls formed on injured almond roots. The presence of plasmid pAgK84, pAgK434, or pAtK84b significantly enhanced the biological control efficacy of K84 derivatives, and the highest level of control was observed with strains harboring two or more plasmids. The results observed with strains deficient in agrocin 434 production suggest that this product may play an important role in the biological control of A. rhizogenes pathogens. The involvement of plasmid pAgK84b in biological control has not previously been reported. This study supports the conclusion that multiple factors are involved in the success of strain K84 as a biological control agent.

Evaluation of Trap Cropping and Biological Control Against Southern Corn Rootworm (Coleopetera: Chrysomelidae) in Peanuts

Field trials to test the efficacy of trap cropping and biological control for the management of Diabrotica undecimpunctata howardi Barber (Chrysomelidae: Luperini) in peanuts were conducted in 1992, 1993, and 1994. Cucurbita maxima Duchesne cv. ‘Blue Hubbard’ was used as a trap crop for adult beetles and the entomopathogenic nematodes, Steinernema carpocapse Weiser and Steinernema riobravis Cabanillas, Poinar and Raulston, were used as biological control agents against soil-inhabiting larvae. In 1992, peanut yields were highest in treatments that included a trap crop. Trap crop did not affect yield in 1993 or 1994. In 2 out of the 3 years, distribution of pod damage relative to the trap crop suggested that beetles oviposited more frequently in peanuts growing in the row next to the trap crop than in peanuts 3 rows from the trap crop. Although entomopathogenic nematodes persisted for a sufficient period to overlap with the presence of rootworms, they did not affect yield or pod damage in peanuts.

Comparison of Entomopathogenic Nematode Dispersal from Infected Hosts Versus Aqueous Suspension

Dispersal of the entomopathogenic nematodes Heterorhabdidtis bacteriophora Poinar (HP88 strain) and Steinernema carpocapsae (Weiser) (All strain) was measured when nematodes were applied to sand either in Galleria mellonella (L.) cadavers or in aqueous suspension. Dispersal ability was estimated as the percentage of nematodes that migrated through sand or from sand onto an agar surface. Results of overnight assays indicated that the dispersal ability of both species was significantly greater when nematodes were applied in cadavers relative to when they were applied in aqueous suspension. Assays that measured migration onto an agar surface after 1 h also indicated enhanced dispersal of S. carpocapsae when exiting cadavers, but results from H. bacteriophora only weakly supported the trend. The relatively greater movement of nematodes exiting infected hosts was not the result of differences in nematode age or persistence in sand. The enhanced dispersal may have been caused by physiological or behavioral differences between nematodes exiting hosts and those kept in aqueous suspension. Because of the dispersal advantage, application of entomopathogenic nematodes in infected hosts may increase their efficacy in biological control. This study demonstrated a need to further investigate behavior of entomopathogenic nematodes when they exit a host under natural conditions.

Interactions of air temperature, relative humidity and biological control agents on grey mold of bean

The interactions ofBotrytis cinerea and seven biological control agents (BCAs) were examined in controlled environments to determine the influence of selected relative humidities (RH) (90,95, and 100%) and air temperatures (20,24 and 28 ‡C) on grey mold of bean. All main effects and interactions were significant (P≤0.05) among the 72 treatments. In the control, lesions of grey mold developed under all environmental conditions but were largest at 24 ‡C×95 and 100% RH, and 28 ‡C×95% RH. Interactions of environment, BCAs and grey mold were complex.Alternaria alternata, Drechslera sp.,Myrothecium verrucaria, Trichoderma viride, Gliocladium roseum and an unidentified pink yeast were all highly dependent on environment for biological control efficacy, and changes of 4 ‡C or 5% RH were associated with variability in disease suppression that ranged from ≤ 15 to 100%. Efficacy ofEpicoccum purpurascens appeared independent of environment and this BCA suppressed disease by 100% in all of the environmental treatments. Suppression of grey mold by many of the BCAs was most effective under environmental conditions least conducive to disease. Therefore, evaluations of potential BCAs in environmental conditions that are marginal for disease can overestimate their efficacy in field environments. Assessments of biological control efficacy in various environments can be used to more accurately assess the potential of BCAs.

Influence of Air Temperature and Relative Humidity on Biological Control of White Mold of Bean (Sclerotinia sclerotiorum)

The interactions of Sclerotinia sclerotiorum and seven biological control agents (BCAs) were examined in controlled environments to determine the influence of selected relative humidities (RH) (90, 95, and 100%) and air temperatures (20, 24, and 28°C) on biological control of white mold of bean. All main effects and interactions were significant (P ≤ 0.0001) among the 72 treatments. In the control treatment, lesions of white mold developed in all environmental treatments but were largest at 20°C x 100% RH, 24°C x 95 and 100% RH, and 28°C x 95% RH. Interactions of environment, BCAs, and white mold were highly complex. Alternaria alternata, Drechslera sp., Myrothecium verrucaria, Trichoderma viride, Gliocladium roseum, and an unidentified pink yeast were all highly dependent on environment for efficacy. Changes of 4°C or 5% RH were associated with variability in disease suppression that ranged from ≤25 to 100%. Epicoccum nigrum was comparatively independent of environment and suppressed disease by 100% in all environments. Suppression of disease by many of the BCAs was most effective under environmental conditions that were least conducive for disease. Assessments of biological control efficacy in various environments can be used to more accurately assess the potential of individual BCAs.

Intraguild Predation Among Biological-Control Agents: Theory and Evidence

Theoretical and empirical evidence developed in four subdisciplines of biological control (biocontrol of plant pathogens, weeds, nematodes, and arthropods) is brought to bear upon a shared question: the significance of intraguild predation. Intraguild predation ("IGP") occurs when two species that share a host or prey (and therefore may compete) also engage in a trophic inter-action with each other (parasitism or predation). We describe the prevalence of IGP and its role in the population dynamics of biological-control agents and target pests. IGP is a widespread interaction within many, but not all, communities of biological-control agents. IGP appears to be pervasive among communities of control agents associated with nematode or arthropod pests. Common forms of IGP include pathogens that infect both herbivores and parasitoids of the herbivore; facultative hyperparasitoids, which can parasitize either an herbivore or a primary parasitoid of the herbivore; predators that attack herbivores that harbor a developing parasitoid; and predators that attach each other. In contrast, IGP appears to be relatively uncommon among biological-control agents of plant pathogens because trophic interactions are less important than competition or antibiosis. Likewise, biological-control agents of weeds interact primarily through competition alone because host ranges are mostly restricted to plant taxa. Empirically based simulation models and general analytical models of interactions involving arthropod pathogens or facultative hyperparasitoids yield variable and often conflicting predictions for the influence of IGP on the success of biological control. Models for predator-predator interactions, however, consistently predict that IGP disrupts biological control. All the field-documented cases of IGP leading to disruption of biological control stem from studies of predators, including mites, insects, and predatory fishes. IGP between two predators or between a predator anti an adult parasitoid does not require mortality of the shared prey/host (i.e., the target pest); thus, IGP can be intense, resulting in high levels of mortality for one or both of the natural enemies, while the total mortality imposed on the target pest population is minimal. For this reason, we hypothesize that IGP by predators is particularly likely to influence the efficacy of biological control. Our ability to develop successful programs of biological control will be enhanced by field studies that address the complexity of trophic interactions occurring in agroecosystems. There is a critical need for additional manipulative experiments conducted in the field that test not only population ecology theory for two-species interactions, but also community ecology theory for multispecies interactions.

Selection of bacterial antagonists for the biological control of Rhizoctonia solani in oilseed rape (Brassica napus)

Spore‐forming bacteria isolated from oilseed rape (OSR) roots were tested for biological control activity against Rhizoctonia solani damping‐off of OSR, using a simple slurry coat formulation. From a total of 239 strains which were tested, Bacillus subtilis 205 was selected as the most promising biocontrol agent (BCA). A known isolate of B. subtilis (NCIMB 12376), which had previously shown activity against R. solani in cotton, was used as a standard comparison throughout. In an in vivo screening, pre‐emergence damping‐off disease control by B. subtilis 205 was excellent, with plant stands being almost as high as non‐diseased controls. However, post‐emergence survival was less good, although the surviving plants exhibited significantly reduced hypocotyl rot. Growth‐room trials which assessed the effect of ambient temperature on the biological control efficacy of B. subtilis strains 12376 and 205 showed that isolate 205 was more effective at lower temperatures. Disease control by isolate 205 at 15°C/10°C (day/night) suggests that this bacterium could operate under autumn sowing field conditions. Further investigations also indicated that the addition of iron (III) to the BCA formulation may enhance disease control by both bacilli. The potential of bacterial BCAs for the control of seedling diseases is discussed.

Quality Control Parameters of Mass-Reared Opiine Parasitoids Used in Augmentative Biological Control of Tephritid Fruit Flies in Hawaii

Two species of opiine braconid parasitoids (Diachasmimorpha longicaudata [Ashmead] and D. tryoni [Cameron]) were mass-reared in insectaries in Oahu, Hawaii and shipped to the island of Kauai for augmentative biological control research and efficacy projects against the oriental fruit fly, Bactrocera dorsalis [Hendel] and Mediterranean fruit fly, Ceratitus capitata [Wiedemann]. Large cohorts of both species were marked with fluorescent dyes to distinguish them from wild parasitoids, and to study their dispersal behavior in the field. Pupal weight, percentage emergence, and sex ratio of dyed and undyed parasitoids were determined for cohorts retained in the insectary, and the combined deleterious effects of transportation and handling on subsequent emergence after field release were measured. The effects of two types of adult parasitoid release vessels on emergence, adult mortality, and short range dispersal were compared, and the negative effects of fluorescent dyes on adult mortality and dispersal were documented. Results are discussed in terms of ways to improve augmentative releases of these opiine parasitoids for fruit fly control.

Results of seven years of biological control of Agrobacterium tumefaciens in Spain1

Biological control of Agrobacterium tumefaciens using the K84 strain of A. radiobacter has been practised in Spain since 1979. Strain K84 is produced in Sevilla by the Agricultural Research Service and is mixed with peat. This formulation is authorized for use on stone fruits and roses, since results of various trials on rootstocks of these crops have shown that K84 is effective in controlling the disease in naturally infected nursery soil, as well as in experimental tests carried out on artificially inoculated soil. Results of biological control trials have shown that K84 has some effect on A. tumefaciens strains resistant to agrocin 84. Factors other than susceptibility to agrocin 84 influence the efficacy of biological control. This was demonstrated by using a K84 agrocin non‐producer strain which proved effective against both resistant and susceptible strains. Biological control on apple or pear is not yet authorized in Spain due to a paucity of data on field trials. However, initial pot tests showed good K84 activity on these hosts, against Spanish strains of A. tumefaciens. On grapevine, most of the A. tumefaciens strains that were isolated belong to biotype 3 and are resistant to agrocin 84. Because the disease is systemic on this species the use of K84 is not authorized.

Susceptibility of the ectoparasite, Bracon mellitor, to infection by microsporidan pathogens in its host, Anthonomus grandis

Theoretical and empirical evidence developed in four subdisciplines of biological control (biocontrol of plant pathogens, weeds, nematodes, and arthropods) is brought to bear upon a shared question: the significance of intraguild predation. Intraguild predation ("IGP") occurs when two species that share a host or prey (and therefore may compete) also engage in a trophic inter-action with each other (parasitism or predation). We describe the prevalence of IGP and its role in the population dynamics of biological-control agents and target pests. IGP is a widespread interaction within many, but not all, communities of biological-control agents. IGP appears to be pervasive among communities of control agents associated with nematode or arthropod pests. Common forms of IGP include pathogens that infect both herbivores and parasitoids of the herbivore; facultative hyperparasitoids, which can parasitize either an herbivore or a primary parasitoid of the herbivore; predators that attack herbivores that harbor a developing parasitoid; and predators that attach each other. In contrast, IGP appears to be relatively uncommon among biological-control agents of plant pathogens because trophic interactions are less important than competition or antibiosis. Likewise, biological-control agents of weeds interact primarily through competition alone because host ranges are mostly restricted to plant taxa. Empirically based simulation models and general analytical models of interactions involving arthropod pathogens or facultative hyperparasitoids yield variable and often conflicting predictions for the influence of IGP on the success of biological control. Models for predator-predator interactions, however, consistently predict that IGP disrupts biological control. All the field-documented cases of IGP leading to disruption of biological control stem from studies of predators, including mites, insects, and predatory fishes. IGP between two predators or between a predator anti an adult parasitoid does not require mortality of the shared prey/host (i.e., the target pest); thus, IGP can be intense, resulting in high levels of mortality for one or both of the natural enemies, while the total mortality imposed on the target pest population is minimal. For this reason, we hypothesize that IGP by predators is particularly likely to influence the efficacy of biological control. Our ability to develop successful programs of biological control will be enhanced by field studies that address the complexity of trophic interactions occurring in agroecosystems. There is a critical need for additional manipulative experiments conducted in the field that test not only population ecology theory for two-species interactions, but also community ecology theory for multispecies interactions.