'Candidatus Liberibacter solanacearum' is a plant pathogen associated with diseases affecting several crops of the Solanaceae and Apiaceae families. Two 'Ca. L. solanacearum' haplotypes (LsoA and LsoB) infect solanaceous crops in North America and are transmitted by the tomato psyllid Bactericera cockerelli. Although both 'Ca. L. solanacearum' haplotypes cause zebra chip in potato, the diseases associated with each haplotype in tomato (Solanum lycopersicum) have not been described. 'Ca. L. solanacearum'-infected tomato plants exhibit symptoms resembling those of permanent yellowing disease (known in Mexico as "permanente del tomate") and sometimes called psyllid yellows. In this study, the symptoms associated with each 'Ca. L. solanacearum' haplotype in tomato were compared, and the bacterial abundance in different nodes of the plants was measured by quantitative polymerase chain reaction. Surprisingly, both plant phenotype and bacterium distribution were different between LsoA- and LsoB-infected plants. Plants infected with LsoB died prematurely, whereas those infected with LsoA did not. Across the measured time points, LsoB abundance in infected plants was consistent with previous reports describing a sink to source gradient, while such gradient was only observed in LsoA-infected plants early after infection. This is the first report describing the differences in symptoms in tomato associated with two 'Ca. L. solanacearum' haplotypes, LsoA and LsoB.

In 2013, an outbreak of Melanaphis sacchari Zehntner (Hemiptera: Aphididae) was reported in sorghum in Texas, USA. Although this aphid has been reported in the continental USA for nearly a century, its occurrence was limited to Florida and Louisiana sugarcane. After 2013 and within just 3 years M. sacchari was reported in almost all sorghum growing regions from south central to southeastern states in the USA. Sorghum fields in affected areas have sustained considerable losses. This aphid has also been reported on Johnsongrass and other feral grasses. The speed at which this aphid has spread raises serious concerns about future infestations. Many aphid species present genetically distinct populations when feeding on different host plants. Thus, it was hypothesized that the recent outbreak in sorghum could be explained by a recent introduction of a sorghum‐specialized genotype. In this study, we genetically characterized M. sacchari in three of its most common host plants – sorghum, sugarcane, and Johnsongrass – across its geographic distribution in the continental USA. Although M. sacchari specimens were grouped within three genetically distinct clusters, we did not find evidence of host plant or geographic population structure. Our characterization of the genetic structure of this pest provides baseline data aimed to help explain its recent outbreak in sorghum, as well as information that may aid in the design of sustainable control strategies.

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The factors explaining host‐associated differentiation (HAD) have not yet been fully characterized, especially in agricultural systems. It is thought that certain characteristics within a system may increase the probability for HAD to occur. These characteristics include relatively long‐standing evolutionary relationships between insects and their host plants, endophagy, and allochrony in host‐plant phenologies. We assessed the status of these characteristics as well as the presence of HAD in the cranberry fruitworm, Acrobasis vaccinii Riley (Lepidoptera: Pyralidae), a pest associated with blueberry and cranberry in eastern North America. We reveal the occurrence of two distinct populations of A. vaccinii that are allochronically isolated by the phenological stage of their respective host plants (cranberries or blueberries). Laboratory‐reared A. vaccinii adults collected from blueberries emerge at least 1 week earlier than adults from cranberries and the antennal sensitivity of adults to host‐plant volatiles differs between A. vaccinii collected from blueberry and cranberry. Despite finding characteristics indicative of HAD, we did not detect a genetic signature of HAD in A. vaccinii. These findings suggest that HAD may occur through behavioral and phenological mechanisms before there is sufficient genetic variation to be detected.

Host‐associated differentiation (HAD) is the formation of genetically distinct, host‐associated populations created and maintained by ecologically mediated reproductive isolation. HAD potentially accounts for species origins in parasites, including herbivorous insects. Although case studies testing the occurrence of HAD are accumulating, it is still unclear how common HAD is and which specific ecological traits explain its occurrence. To address these issues, studies are needed that include negative results (i.e., instances in which parasite populations do not exhibit HAD) and test for HAD across communities (i.e., several parasite species on the same set of host species). In this study, HAD was tested in a community of six species of Aphidomorpha (Hemiptera) that share a host‐plant pair: pecan [Carya illinoinensis (Wangenh.) K.Koch] and water hickory [Carya aquatica (F.Michx) Nutt., both Juglandaceae] trees. All six species are parthenogenetic and three species are endophagous, traits that can exacerbate host‐specific selection. AFLP markers were employed to detect the presence of genetically distinct, host‐associated populations for each insect species. Strict HAD (i.e., the occurrence of genetically distinct pecan‐associated and water hickory‐associated genotypes) was found in Phylloxera notabilis Pergande (Phylloxeridae), Phylloxera devastatrix Pergande, and Monelliopsis pecanis Bissel (Aphididae). Monellia caryella Fitch (Aphididae) displayed a pattern of partial HAD (i.e., the occurrence of only a genetically distinct pecan‐associated genotype). No HAD was found in Melanocallis caryaefoliae Davis (Aphididae) or Phylloxera texana Stoetzel. The pattern of HAD occurrence in the pecan and water hickory Aphidomorpha community indicated that neither parthenogenesis nor endophagy sufficiently explain the occurrence of HAD in this system.