Specificity and host specialization of Puccinia chondrillina

Various strains of Puccinia chondrillina, the Chondrilla rust, were collected during surveys in the Mediterranean, undertaken to discover biological control agents suitable for use in Australia against skeleton weed, Chondrilla juncea. Only the strain collected at Vieste (S.E. Italy) was highly virulent against the common Australian form of C. juncea. The specificity of this strain of the rust was tested by inoculation of many unrelated cultivated plants, related cultivated plants and closely related wild plants; all were immune to the rust. These tests satisfied the Australian plant quarantine authorities and this strain of the rust has now been introduced into Australia as a biological control agent for C. juncea.     

Biology and Evolution of Beneficial and Detrimental Viruses of Animals, Plants, and Fungi

Virtually every type of organism may serve as a host for viruses. In some hosts, virus presence may be considered beneficial to humans; in other hosts, viruses are considered detrimental. Examples of viruses that are considered beneficial to humans include those that are used for biological control of organisms that themselves are considered detrimental to humans, such as plant pathogenic fungi. Viruses are extremely variable in terms of morphology, structure, and genome organization. However, viruses that attack hosts from different kingdoms may be related, deriving from the same phylogeny. This paper summarizes some of the properties of three related families of viruses that attack hosts in different kingdoms: the animal-infecting Picornaviridae, the plant-infecting Potyviridae, and the fungus-infecting Hypoviridae. Properties of these viruses that set them apart from each other and factors that may affect their evolution are discussed.     

The effect of host plant intraspecific genetic variation on the fitness of a monophagous biological control agent

Variation in insect life-history traits when developing on different host plant genotypes with known genetic relationships was tested using the monophagous biological control agent Phenrica guérini Bechyné (Chrysomelidae) and its host plant, Pereskia aculeata Miller (Cactaceae). Differences in insect fitness among tested host plant genotypes were expected because of geographic isolation and large genetic distances between plant genotypes, and because P. guérini has been associated with some of the plant accessions in the field while others have never been exposed to herbivory by the insect. There was little variation in insect life-history traits among plant genotypes and no differences in the insect's ability to utilise different plant genotypes. The results of the study suggest that, in some cases, biological control agents collected from the native genotype most closely related to the target weed population will not always be more effective than those collected from more distantly related genotypes.     

Plant neighbor identity influences plant biochemistry and physiology related to defense

Background  

Chemical and biological processes dictate an individual organism's ability to recognize and respond to other organisms. A small but growing body of evidence suggests that plants may be capable of recognizing and responding to neighboring plants in a species specific fashion. Here we tested whether or not individuals of the invasive exotic weed, Centaurea maculosa, would modulate their defensive strategy in response to different plant neighbors.     

Exotic wildland weeds serve as reservoirs for a newly introduced cole crop pest,Bagrada hilaris (Hemiptera: Pentatomidae)

The role of wildland weeds that serve as alternate hosts for insect pests has not been directly examined for the potential to sustain pest populations or facilitate pest outbreaks. The development of weed biological control programmes is also complicated by weed species that are closely related to economically important or native plants, especially rare or special status species. In recent surveys in southern California, USA, we found a newly introduced insect pest of cole crops, Bagrada hilaris Burmeister (Bagrada bug), feeding on Brassicaceae weeds in riparian areas adjacent to agricultural fields where cole crops are routinely grown. Insect populations grew to levels well over action thresholds and caused severe damage to populations of the invasive weed, Lepidium latifolium (perennial pepperweed). The numerical response of B. hilaris on L. latifolium and other Brassicaceae weeds in natural areas may pose a significant challenge to effectively managing pest populations in crops. However, the accidental introduction of this insect provides the opportunity to examine plant–insect interactions with important implications for development of biologically based control methods for weeds.     

Effect of Coleus Size and Variegation on Attack Rates, Searching Strategy, and Selected Life History Characteristics of Cryptolaemus montrouzieri (Coleoptera: Coccinellidae)

Twenty-four-hour attack rates and the search strategy of third instar Cryptolaemus montrouzieri Mulsant (Coleoptera: Coccinellidae) attacking 1 to 16 third instar Planococcus citri Risso (Homoptera: Pseudococcidae) were measured on green and yellow-variegated Solenostemon scutellarioides (L.) Codd (= Coleus blumei (Bentham)) (Labiatae) plants of different sizes. Selected life history characteristics of C. montrouzieri fed different amounts of P. citri as prey from third instar to adults were also examined. On average, predators attacked 1 to 4 mealybugs, depending on the number of mealybugs and plant size. There was no effect of plant color on attack rates. Attack rates were positively related to prey density, whereas the estimated area searched by predators was inversely related to prey density. Analyses suggest that leaf area was the plant characteristic that most affected attack rates. Predators fed few prey had a decrease in body weight and survival. The implications for the use of C. montrouzieri in biological control are discussed.     

You stay,but I Hop: Host shifting near and far co‐dominated the evolution of Enchenopa treehoppers

The importance and prevalence of phylogenetic tracking between hosts and dependent organisms caused by co‐evolution and shifting between closely related host species have been debated for decades. Most studies of phylogenetic tracking among phytophagous insects and their host plants have been limited to insects feeding on a narrow range of host species. However, narrow host ranges can confound phylogenetic tracking (phylogenetic tracking hypothesis) with host shifting between hosts of intermediate relationship (intermediate hypothesis). Here, we investigated the evolutionary history of the Enchenopa binotata complex of treehoppers. Each species in this complex has high host fidelity, but the entire complex uses hosts across eight plant orders. The phylogenies of E. binotata were reconstructed to evaluate whether (1) tracking host phylogeny; or (2) shifting between intermediately related host plants better explains the evolutionary history of E. binotata. Our results suggest that E. binotata primarily shifted between both distant and intermediate host plants regardless of host phylogeny and less frequently tracked the phylogeny of their hosts. These findings indicate that phytophagous insects with high host fidelity, such as E. binotata, are capable of adaptation not only to closely related host plants but also to novel hosts, likely with diverse phenology and defense mechanisms.     

Maintenance of a narrow host range by Oxyops vitiosa; a biological control agent of Melaleuca quinquenervia

Host range expansion in insect herbivores is often thought to be mediated by several factors, principal among them are secondary plant metabolites. In weed biological control, the host range of a prospective agent is one of the most important considerations in its implementation. Extensive host testing tests seek to determine the behavioral acceptance and nutritional value of different test plant species to the potential agent. A list of test plants is compiled that comprises species that are close taxonomic relatives of the target weed plus other species of economic or ecologic importance. The host testing of the Melaleuca quinquenervia biological control agent Oxyops vitiosa indicated that larvae would accept and complete development on the Australian target weed M. quinquenervia, two Australian ornamental species, Callistemon citrina, Callistemon viminalis (all Myrtaceae). However, the larvae did not complete development when fed a North American species Myrica cerifera (Myricaceae). The study reported here confirms these results and examines the nutritional and performance differences in O. vitiosa larvae fed leaves of these species. The leaf quality factors, percent moisture, percent nitrogen, toughness, and terpenoid content were related to larval survival, performance and digestive indices. The results indicate that plant quality among the Myrtaceae species was generally similar and correspondingly larval survival, performance and digestive indices differed little when larvae were fed leaves of these species. However, significant differences occurred in the plant quality of the North American M. cerifera compared with the Australian species which had leaves with the lowest percent moisture, lowest leaf toughness, highest percent nitrogen. This species, however, is not a physiological host as none of the neonates survived to pupate. When third instars were switched to M. cerifera from their normal host M. quinquenervia reductions were found in survival, biomass gain, digestive efficiency, and conversion of digested food to insect biomass. The marginal acceptance of this North American native plant in laboratory bioassays appears related to the terpenoid chemistry that has similarities to the taxonomically unrelated host M. quinquenervia. However, the high larval mortality corresponds to several novel terpenoids that are not present in the host. For weed biological control host testing these results indicate that M. cerifera is a poor host for O. vitiosa. Additionally, future test plant lists should include plants with secondary metabolites similar to the target weed as these compounds may constitute behavioral cues that are relevant to these specialized herbivores.     

A two-phase open-field test to evaluate the host-specificity of candidate biological control agents for Heliotropium amplexicaule

This paper describes an open-field host-specificity test conducted to make a preliminary evaluation of the safety of four candidate agents for the biological control of Heliotropium amplexicaule, an invasive weed of South American origin. These agents were a leaf-eating beetle, Deuterocampta quadrijuga, a flea-beetle, Longitarsus sp., that feeds on leaves as an adult and roots as a larva, a shoot-feeding thrips, Haplothrips heliotropica, and a cell-sucking bug, Dictyla sp. During the first phase of the experiment, the four agents were given a choice between the target weed and six species of nontarget plant of varying degrees of phylogenetic relatedness. All four species were found to feed and reproduce on only H. amplexicaule and the closely related H. nicotianaefolium (a member of the same section of the genus Heliotropium). No plants outside this genus were attacked. For the second “no-choice” phase, the host-plants used in Phase 1 were cut, forcing the insects to use either other plant species within the plots, emigrate, or die of starvation. Heliothrips heliotropica disappeared rapidly from the plot, while D. quadrijuga persisted for several days on Heliotropium arborescens with some exploratory nibbling and then disappeared. Host-choice behavior of these species therefore remained unchanged, even in the absence of the primary host-plants. In contrast, adults of Longitarsus sp. rapidly colonized and fed on H. arborescens when the preferred hosts were removed. It therefore demonstrated a wider host-range under these more extreme conditions. In fact, on some plots, feeding commenced on H. arborescens before the “no-choice” phase, once the two preferred Heliotropium species were heavily damaged by these insects. The two-phase test is shown here to be a useful way of measuring host-choice behavior under “normal” conditions of choice and under more extreme conditions, such as it might occur were an agent to locally destroy the target weed following its release. It therefore provides a more refined assessment of the risk that would be posed to nontarget plants by the release of weed biological control agents.     

Host Specificity of the Argentine Root-Boring Weevil, Heilipodus ventralis (Coleoptera: Curculionidae), a Potential Biocontrol Agent for Snakeweeds (Gutierrezia: Asteraceae) in Western North American Rangelands—U.S. Quarantine Tests

Native snakeweeds, especially Gutierrezia sarothrae (Pursh) Britton and Rusby and Gutierrezia microcephala (DC.) A. Gray, are among the most widespread and damaging weeds of rangelands in the western United States and northern Mexico. The genus long ago spread to southern South America, where further speciation occurred. We have found several species of insects in Argentina that damage other species of snakeweeds there and are possible candidates for biological control in North America. The first of these, the root-boring weevil, Heilipodus ventralis (Hustache), was tested in Argentina and then sent to the USDA-ARS Insect Quarantine Facility at Temple, Texas, for host specificity testing on North American plants. We tested H. ventralis on 40 species of the family Asteraceae, in 19 tests of five types, using 686 adults and 365 larvae. Host specificity increased from adult feeding, to ovipositional selection, to larval development. At Temple, adults fed mostly on 6 species of the closely related genera Grindelia, Gutierrezia, and Gymnosperma, but with substantial feeding on four other genera of the two preferred subtribes Solidagininae and Machaerantherinae and on Baccharis in the tribe Baccharidinae, with lesser feeding on the subtribe Asterinae, all in the tribe Astereae, and on 1 species in the tribe Anthemideae. Females oviposited primarily on the same 6 species but very little on plants outside the 2 preferred subtribes. Larvae developed only on 9 of the 29 U.S. plant species tested, 6 within the two preferred subtribes and on Brickellia and Aster in other tribes. Only 5 species of three genera appear to be potential true hosts of H. ventralis in North America, on which all stages of the life cycle, adult feeding, oviposition, and larval development, can take place; these are Gymnosperma glutinosum (Spreng.) Less., Gutierrezia grandis Blake, Gut. microcephala, Gut. sarothrae, and Grindelia lanceolata Nutt. None of these genera contain species of economic or notable ecological value; the few rare species appear to be protected by habitat isolation from attack by H. ventralis. H. ventralis, therefore, appears sufficiently host specific for field release in North America. This is the first introduced biocontrol agent to be approved for release in a continental area to control a native weed.     

Plant size preference of Agapeta zoegana L. (Lepidoptera: Tortricidae), a root-feeding biological control agent of spotted knapweed

Agapeta zoegana L. (Lepidoptera: Tortricidae) is an oligophagous herbivore that was introduced to North America as a biological control agent of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek (often called Centaurea maculosa Lam.). Spotted knapweed is a perennial plant that usually increases in size each year. A previous field study reported that more larvae were found on larger plants and that infested plants tended to be larger than uninfested ones. Precisely quantifying the size-specific attack rate can help us model the impact of this agent on the weed population and better understand the interspecific interactions to improve the effectiveness of biological control. Field data were analyzed to determine the relative preference of attack for each size class of the host plant. Plants were classified based on root diameter at 2 cm below the root crown. Although small plants (<3 mm root diameter) were more abundant in the field population, the highest infestation rates occurred in large plants. Chesson’s electivity index was generally positive for root diameters >3.5 mm, indicating preferential attack of large plants. Because of its host-size preference, A. zoegana is expected to primarily affect large plants, which is contrary to previous expectations. Quantifying the insect’s direct impact is a difficult challenge, which may require several field seasons of measuring accumulated damage on individual plants. In order to complement the biological control agents already established, foreign exploration should focus on finding an agent that attacks young knapweed plants. These results also indicate that the efficiency of sampling roots in the field to detect the presence of A. zoegana can be improved by choosing only the largest plants.     

Interactive effects of light environment and herbivory on growth and productivity of an invasive annual vine, Persicaria perfoliata

Plant populations often exist in spatially heterogeneous environments with varying light levels, which can affect plant growth directly through resource availability or indirectly by altering behavior or success of herbivores. The plant vigor hypothesis predicts that herbivores are more likely to attack vigorously growing plants than those that are suppressed, for example in more shaded conditions. Plant tolerance of herbivory can also vary under contrasting resource availability. Observations suggest that damage by Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae), introduced into the United States in 2004 as a biological control agent for mile-a-minute weed (Persicaria perfoliata [L.] H. Gross), is greater in the sun than in shade. We compared weevil densities and plant growth in paired plots in full sun or under shade cloth; a second experiment included insecticide-treated plots in sun and shade, to assess the ability of the plant to compensate for herbivore damage. Greater density of weevils and more node damage (indicating internal larval feeding) were found on P. perfoliata plants growing in sun than on those in shade. Nodes were 14% thicker in the sun, which may have provided better larval habitat. Biomass produced by plants without weevils in the sun was about twice that produced in any other treatment. Herbivory had a greater effect on plant growth in the high-light environment than in the shade, apparently because of movement into the sun and increased feeding there by the monophagous herbivore, R. latipes. Results support the plant vigor hypothesis and suggest that high weevil densities in the sunny habitats favored by P. perfoliata can suppress plant growth, negating the resource advantage to plants growing in the sun.     

Predictable risk to native plants in weed biological control

Data on field host use of 112 insects, 3 fungi, 1 mite, and 1 nematode established for biological control of weeds in Hawaii, the continental United States, and the Caribbean indicate that the risk to native flora can be judged reliably before introduction. Virtually all risk is borne by native plant species that are closely related to target weeds. Fifteen species of insects introduced for biological control use 41 native plant species; 36 of which are congeneric with target weeds, while 4 others belong to two closely allied genera. Only 1 of 117 established biological organisms uses a native plant unrelated to the target weed. Thus the elements of protection for the native flora are the selection of weed targets that have few or no native congeners and the introduction of biological control organisms with suitably narrow diets.     

Field assessment in land of origin of host specificity,infestation rate and impact of <Emphasis Type="Italic">Ceratapion basicorne</Emphasis> a prospective biological control agent of yellow starthistle

Yellow starthistle, Centaurea solstitialis (Asteraceae), is an important invasive alien weed in the western United States. Currently established biological control agents attack only the capitula (flowerheads), and are not effectively controlling the plant in much of its range. The geographic center of diversity for the plant appears to be in Turkey, but no agents have been introduced from this country. Ceratapion basicorne (Coleoptera: Apionidae) is common in Central Turkey, attacking 25–100% of yellow starthistle plants. In a field experiment, Ceratapion spp. attacked 90% of yellow starthistle plants and 88% of milk thistle plants (Silybum marianum) but not seven other plant species, including artichoke and safflower. We suspect that a different species of insect attacked milk thistle, but they emerged before the plants were sampled. Laboratory tests showed that C. basicorne does not oviposit in milk thistle. Ceratapion basicorne appears to be more host specific than was suggested by previous studies of a population in Italy (Clement etal. 1989. Ann. Entomol. Soc. Am. 82: 741–747). The insect is gregarious, and the number of larvae per plant was positively correlated to root diameter. The level of damage to individual plants was positively correlated to the proportion of plants attacked, indicating aggregation both among plants and within plants. Field data did not show any impact of the insect on plant size or number of capitula, but germination rate of seeds produced by infested plants was 15% lower than for uninfested plants at two of three sites studied.     

Field host-specificity of the mile-a-minute weevil,Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae)

The safe practice of biological control relies, in part, on an accurate evaluation of a potential agent’s host-specificity via testing through a “filter of safety”. The results of laboratory tests may differ from those obtained in open field host-specificity tests, where agents are able to use their full range of host-selection behaviors. It was hypothesized that Rhinoncomimus latipes (Coleoptera: Curculionidae), the biological control agent released against mile-a-minute weed, Persicaria perfoliata (Polygonaceae), would not feed or oviposit on nontarget plants in a two-phase, open field setting. Ten weevils were placed at the base of each of 13 test plant species in a randomized complete block design with six replicates. Weevils placed at the base of mile-a-minute weed were marked with yellow fluorescent dust, and yellow weevils were subsequently found only on mile-a-minute. Weevils placed at the base of nontarget plants (marked with red fluorescent dust) rapidly colonized mile-a-minute weed. Three hours after release, the number of R. latipes found on mile-a-minute weed was significantly higher than predicted by a random distribution of weevils on all test plants. The likelihood of finding more weevils on mile-a-minute compared to nontarget plant species was 31.0% at 3 h and increased to 96.5% at 44 h after release. Whereas prerelease studies showed feeding at low levels on 9 of the 13 plant species tested here, under open field conditions R. latipes did not feed on any nontarget plant species and dispersed from these plants. In an open field setting, where the weevil was able to use its full range of host-selection behaviors, there was no observed risk of nontarget effects for any species tested.     

Field assessment of the risk posed by Diorhabda elongata, a biocontrol agent for control of saltcedar (Tamarix spp.), to a nontarget plant, Frankenia salina

The biological control program for saltcedar (Tamarix spp.) has led to open releases of a specialist beetle (Chrysomelidae: Diorhabda elongata) in several research locations, but the controversy over potential impacts to native, nontarget plants of the genus Frankenia remains unresolved. To assess the potential for nontarget impacts under field conditions, we installed cultivated Frankenia spp. (primarily two forms of Frankenia salina but also including Frankenia jamesii) at locations in Nevada and Wyoming where D. elongata densities and saltcedar defoliation were expected to be very high, so insects would be near starvation with high probability of attacking nontargets if these were suitable hosts. Subsequent insect abundance was high, and only minor impact (<4% foliar damage) was observed on both forms of F. salina under these ‘worst case’ conditions; there was no impact to F. jamesii. No oviposition nor larval development were observed on any plants, there was no dieback of damaged F. salina stems, and plants continued growing once insect populations subsided. These results under ‘natural’ field conditions contrast with caged host-range tests in which feeding, development and minor oviposition occurred on the nontarget plant. Other ecological factors, such as distance from target plants to natural Frankenia spp. populations, inhospitable conditions for agent survival in such sites, and intrinsic insect behavior that makes colonization and/or genetic adaptation highly unlikely, lead us to conclude that nontarget impacts following program implementation will be insignificant or absent. Host range testing of new agents, while necessary to ensure safety, must put greater attention on assessing the ecological context where agents will be establishing, and on balancing speculated risks against potential benefits of biological control.     

Biology, Impact and Preliminary Host-specificity Testing of the Rust Fungus, Uromyces salsolae , a Potential Biological Control Agent for Salsola kali in the USA

Russian thistle, Salsola kali (=Salsola australis) (Chenopodiaceae) of Eurasian origin is a troublesome weed in the drier regions of the western USA. The weed commonly infests crops, disturbed wastelands and overgrazed rangelands. A rust fungus, Uromyces salsolae , has been found to cause considerable damage to the weed in Eurasia. Field observations in Turkey revealed that S. kali plants infected by the rust were covered with a powdery brown mass of urediniospores and had stunted growth. Under laboratory conditions, U. salsolae severely infected Salsola plants from France, the USA and Turkey (control). The urediniospores germinated when in contact with water within a minimum period of 2 h and over a wide range of temperatures. S. kali plants were also infected at a wide range of temperatures and incubation periods with simulated dew. Fungal attack was very severe and caused mortality or much reduced growth of infected plants without seed production. Preliminary host specificity testing using 17 plant species or varieties from six families revealed that the rust infected only the Russian thistle. U. salsolae was imported recently into the USA for further host specificity testing under quarantine conditions. Its use as a biological control agent for S. kali in the USA is recommended.     

Strategy differences of two potato species in response to nitrogen starvation. Do plants have a genetic switch for nitrogen signalling?

Survival responses to nitrogen starvation are well known in micro‐organisms but little studied in plants. To construct a framework for study of the plant responses, we investigated the strategy differences of tubers from two closely related potato species. Solanum tuberosum conserves tuber nitrogen by inhibiting shoot growth, but S. phureja mobilizes tuber nitrogen to grow shoots, flowers and seeds. Genetic analysis of progeny from S. phureja–haploid S. tuberosum crosses uncovered segregation of a single dominant gene for the S. tuberosum inhibition strategy. Within S. tuberosum, haploid progeny closely resembled their tetraploid parents, suggesting strong genetic control of the inhibition. Growth of the inhibited shoots was proportional to sub‐optimal levels of added nitrate, and was triggered by exogenous gibberellic acid (GA₃). These observations support the notion that potato plants can closely tie shoot growth to ambient nitrogen levels – probably by a root–shoot nitrogen signal transduction pathway, and that this can be overridden by emergency mobilization of nitrogen reserves, perhaps by GA signalling from the tuber. Furthermore, genes for such developmental switches can be identified by classical genetic analysis of closely related species, such as S. tuberosum and S. phureja, that exhibit opposite survival strategies.     

Prospects for biological control of teasels,Dipsacus spp., a new target in the United States

Two closely related teasels (Dipsacales: Dipsacaceae, Dipsacus spp.) of European origin have become invasive weeds in the United States. Common teasel (Dipsacus fullonum L.) and cutleaf teasel (Dipsacus laciniatus L.) have likely been in North America for more than two centuries, having been introduced along with cultivated teasel [D. sativus (L.) Honckney], an obsolete crop plant. There are few records of American insects or pathogens attacking Dipsacus spp. Invasive teasels have recently begun to spread rapidly throughout much of their current range, for reasons that are not yet known. Common and/or cut-leaf teasel have been listed as noxious in five US states and as invasive in 12 other states and four national parks. Because the family Dipsacaceae is an exclusively Old World family, classical biological control is an important component of the overall management strategy of this weed in the US. Field surveys for natural enemies of D. fullonum and D. laciniatus in their native ranges and literature reviews of natural enemies of plants in the family Dipsacaceae have yielded 102 species of insects in six orders, as well as 27 fungi from 10 orders, three mites, one nematode, and two viruses. Due to the biennial nature of these weeds, a strategy to assign highest priority to biological control candidates attacking first-year (rosette) plants has been established. Candidates selected for further study based on this strategy include Chromatomyia ramosa (Hendel) (Diptera: Agromyzidae), Longitarsus strigicollis Wollaston (Coleoptera: Chrysomelidae), Epitrimerus knautiae Liro (Acarina: Eriophyiidae), Euphydryas desfontainii (Godart) (Lepidoptera: Nymphalidae), Erysiphe knautiae Duby (Erysiphales: Erysiphaceae), and Sphaerotheca dipsacearum (Tul. and C. Tul.) (Erysiphales: Erysiphaceae).     

Cover Caption

Foliar nutrient content plays a key role in mediating the interaction of plants and insect herbivores within and across generations. Here, the weed biological control agent Neolema abbreviate is depicted feeding on its host Tradescantia fluminensis during trials measuring performance across generations on hosts with variable nitrogen content. Our study concluded that adult traits such as oviposition selection, weigh gain and longevity were largely unresponsive to parental diet. Instead, offspring diet was more important in determining performance (see pages 361–374). Photo provided Frank Chidawanyika.