The chestnut gall wasp (CGW), Dryocosmus kuriphilus Yasumatsu, 1951 (Cynipini, Cynipoidea, Hymenoptera), is a globally invasive pest on chestnuts (Castanea spp., Fagaceae). Since its first reported occurrence in the state of Georgia in 1974, CGW has been reported to have spread to 16 additional states in the eastern United States. Despite its wide occurrence, investigations of the invasion and colonization history of CGW have not been conducted. To address this shortcoming, we screened one mitochondrial (cytochrome c oxidase subunit 1; COI) and one nuclear gene (internal transcribed spacer 2; ITS2) to examine the genetic variation across four discrete CGW populations from four states (New Jersey, Pennsylvania, Maryland, Tennessee). We tested whether CGW in the eastern U.S. originated from one or multiple introductions followed by subsequent dispersal. Additional insights came from comparing the sequences with two CGW populations sampled from its native range in mainland China, and one population each from Taiwan and Japan. Sequences revealed that both genes exhibit the same haplotype across all sampled specimens of all locations. The COI haplotype in this study is the dominant haplotype recorded in China, while the haplotype of ITS2 has been reported in both Japan and Europe. The uniformity of genetic structure suggests that CGW invasion in the U.S. could have either single or multiple origins, or both, accompanied with the common haplotype on each occasion. Further insights into the invasion history and expansion of CGW in the U.S. will require a combination of sampling more populations and utilizing a next generation sequencing strategy.

Larvae of Lopesia curupirae sp. nov., a new species of Cecidomyiidae (Diptera), were found in leaflet galls of Anadenanthera colubrina (Vell.) Brenan (Fabaceae) sampled in an urban park in Ribeirão Preto municipality, São Paulo State, Brazil. In this paper, we describe and illustrate the larva, pupa, male, female, gall, and aspects of biology of the new species.

Galls and gall-inducing insect inventories have allowed us to understand patterns of species richness and differential distribution, and to map interactions between gall-inducing insects and host plants in different Brazilian biomes. Since the first formal inventory of gall-inducing insects, conducted in 1988 by Fernandes and collaborators in the Cerrado biome in Minas Gerais, studies on this topic have continued to increase. Over the past 35 years, there have been 64 inventories in over 185 locations, spanning 114 municipalities in 18 states across the five regions of Brazil, and involving 97 researchers. Despite the advances in recent decades, between 1988 and 2023, there has not been a significant increase in the number of publications. This is mainly due to the decrease in the number of papers published in the last years (2014–2023). We constructed a co-authorship network that shows some authors as important hubs in collaborations between researchers and institutions; however, some still exhibit low penetration and integration in the research network. The results showed that (i) there is a predominance of studies in southeast Brazil, in which predominates the Atlantic Forest and Cerrado; (ii) gall-inducing insects are abundant in any environment but dominant in sclerophyll conditions; (iii) gall-inducing insect richness is positively associated with host plant species richness; (iv) Fabaceae is the richest host family; and (v) Cecidomyiidae (Diptera) are the main gall inducers. In Brazil, 284 species of cecidomyiids, most of which induce gall formation, have been recorded. The majority of species have been described in the Atlantic Forest (184), followed by the Cerrado (70) and the Amazon Forest (31). However, it is important to note that there may be bias in the distribution of knowledge and a significant taxonomic gap that needs to be addressed. These results provide valuable insights for future research.

Cecidomyiinae, by far the largest subfamily in the family Cecidomyiidae, may owe their success as plant feeders and predators largely to larval adaptations that were already present in the most primitive cecidomyiids that fed on fungi in decaying organic matter. These modifications include miniaturization of the head and body, the change from chewing to piercing-sucking mouthparts, and the spatula, a dermal structure unique to the family on the prothoracic segment of the mature larva. Presented here are examples that illustrate these attributes of cecidomyid larvae, chiefly in their role as plant feeders. Concurrent changes in pupal morphology are also noted.

To contribute to the understanding of the coevolution of the gall-inducing tephritid fly, Eurosta solidaginis Fitch, 1855 (Diptera: Tephritidae), and its host plant, tall goldenrod, Solidago altissima L. (Asteraceae), we tested hypotheses on how plant genotypic variation influenced gall size and shape. A field survey showed that stem diameter was correlated with gall diameter, indicating that plant phenotype influences gall size. To test the hypothesis that plant genotypic variation in stem growth influenced gall diameter, length, and shape, 10 goldenrod genotypes were grown in a common garden and exposed to flies in a cage in 2019 and 2020. Plant genotype significantly influenced gall diameter and length in both years and gall shape in the second year of the experiment. Our results support those of Weis & Abrahamson (1986), who found that S. altissima gall diameter results from the interaction of plant genotype, insect genotype, and the environment. Neither stem diameter nor length significantly impacted gall diameter, length, or shape in the common garden experiments, suggesting plant genotype influences these traits through other factors uncorrelated with stem growth. Because genetic variation in gall size is weakly correlated with stem growth, this removes restraints on plant response in the evolution of gall size in response to selection by Eurosta.

Gall-inducing insects engage in intricate interactions with their host plants, significantly affecting ecosystem health. Invasive gall-inducing insects, however, pose detection challenges and can cause significant, often irreversible damage. Citizen science may play an important role in establishing an integrated framework and conducting comprehensive monitoring of such invasive species. We propose utilizing the iNaturalist platform to gather and systematize these interactions through a two-step method aimed at assessing the distribution of invasive gall-inducing insects and their host plants. The widespread global gall-inducing insects, i.e., Ophelimus maskelli (Ashmead, 1900) and Leptocybe invasa Fisher & La Salle, 2004 (Hymenoptera: Eulophidae), were selected as targets for modeling. The first step involves gathering and establishing the connections between gall-inducing insects and their host plants, followed by performing a spatial distribution prediction based on niche modeling. Subsequently, we conduct a thorough analysis of the existing literature and examine various physiological factors. Then, we aim to develop potential distribution models that allow us to assess the extent of endangerment posed to ecosystem health. The distribution of invasive gall-inducing species on Eucalyptus L'Hér. (Myrtaceae) plantations have been revealed through observations on the iNaturalist platform. Notably, there has been an expansion towards higher latitudes observed at certain sites. Citizen participation enriches research and enhances public scientific knowledge, which is vital for informing conservation policies and implementing green subsidies.

Outbreaks of gall inducers sometimes cause serious damage to their host plants. In recent decades, the gall midge Schizomyia castanopsisae Elsayed & Tokuda, 2018 (Diptera: Cecidomyiidae) inducing inflorescence galls on Castanopsis sieboldii (Makino) Hatus. (Fagaceae) occurred at extremely high densities in southern Izu Islands, Japan, and led to severe loss of host acorn production. This gall midge is suspected to have recently invaded the Izu Islands from the Nansei Islands, and the absence of associated parasitoids may cause the outbreaks there. Although this gall midge was not found in the northern Izu islands nor in the Izu Peninsula in early 2010, northward range and population expansions have been suspected in later surveys. In this study, we investigated the galls of S. castanopsisae in northern Izu islands and compared the changes in population density in this decade. As a result, the density of galls was noticeably increased on Ohshima, Niijima, and Kozushima Islands. Parasitoids were not found in galls collected from these islands. We consider that the outbreaks of S. castanopsisae are gradually expanding northward in the Izu Islands.

Megachile montivaga Cresson, 1878 is a solitary bee that has been documented to nest in the soil, trap nests, and in the stems of several different plant species within its native range in North America. In this study, M. montivaga is shown for the first time to nest in goldenrod ball galls of Solidago altissima L. (Asteraceae). Adults of M. montivaga were reared from ball galls that have a characteristic basal surface excavation that serves as the nest entrance. A survey of 6702 goldenrod ball galls was conducted at 31 sites in three Mid-Atlantic states to gain a better understanding of the frequency with which M. montivaga nests in goldenrod ball galls. Ball galls with basal surface excavations characteristic of nest entrances for this species were found at 14 field sites in Pennsylvania, New Jersey, and New York in this study. Goldenrod ball galls with nests yielded five adult bees of M. montivaga; six additional galls had definitive evidence of bee nesting activity as they contained bee nesting materials in brood chambers. A limited number of bee species have previously been shown to nest in other types of insect galls; herein, we provide a review of what is already known about this interesting phenomenon. Prior studies have documented that selected species of bees can nest in particular types of abandoned insect galls, while this study provides evidence that M. montivaga can form nests in living, occupied ball galls of S. altissima.

Plant galls are thought to evolve under natural selection as extended phenotypes of the insects that induce them. An evolutionary response to selection requires that the phenotypic variance in the selected trait be caused by underlying heritable genetic variance. We used a one-generation artificial selection experiment to verify heritable variation in the size of the gall induced by the tephritid Eurosta solidaginis (Fitch, 1855) on the stems of tall goldenrod, Solidago altissima L. (Asteraceae). Previous work under greenhouse conditions demonstrated that gall size is a heritable trait of species of Eurosta Loew, 1873, but two questions remain. Foremost, is the genetic signal that is evident in a uniform, controlled environment still detectable in heterogenous natural environments? And secondarily, since galls initiated later in the season are known to grow to a smaller size, is genetic variance in gall size attributable to genetic variance in Eurosta phenology? We imposed upward and downward selection on size by populating isolated goldenrod patches with parental flies emerging from either large diameter (> 24 mm) or small diameter (< 17 mm) or galls. Galls produced by the offspring in upward-selected patches were ∼19% larger than those in downward sites. While gall size declined with later initiation dates, there was no evidence that the upward- and downward-selected sites varied in either oviposition date or gall initiation date; thus, genetic variance in gall size is not rooted in genetic variance for phenology. Our results support the notion that while the gall is plant tissue, it develops under the influence of the insect's genotype, and thus can evolve as the gallmaker's extended phenotype.

Fire is a major ecological process maintaining chaparral ecosystems in California. Following a 1999 wildfire at our study site in the Cascade Range foothills, we have tracked the appearance of young manzanita, Arctostaphylos Adanson (Ericaceae), plants and their colonization by Tamalia Baker, 1920 (Aphididae) aphids beginning in 2003; hence, we have collected data on rates of colonization by the gall-inducers, Tamalia coweni (Cockerell, 1905), and their inquilines, Tamalia inquilinus Miller, 2000. Our methods included mapping the spatial distribution of > 500 host plants in a 1-ha study population using a high-precision Trimble® global positioning system (GPS) instrument and a Geographic Information System (GIS) to process data. We surveyed juvenile shrubs and a random sample of mature plants to estimate the frequency and timing of plants colonized by T. coweni. Additionally, we sampled galls over the growing season to estimate the frequency of inquilines occupying galls. Beginning in 2008, less than 4% of the 135 juvenile plants were colonized by T. coweni as evidenced by the presence of new galls. The proportion of plants colonized has increased continuously; over 85% had been colonized by 2023. Assuming T. coweni colonizes young plants (sinks) from existing populations on mature plants (sources), our results may conform to a minimum dispersal distance hypothesis, although this remains to be tested explicitly. Our data further suggest T. inquilinus can disperse and colonize new habitats efficiently, in synchronization with T. coweni. Our results have implications for patterns of evolutionary diversification in both gall-inducer and inquiline lineages.