Effect of temperature on the population dynamics of three Dalbulus leafhopper species

Adult survival and fecundity of three Dalbulus leafhopper species were determined at constant temperatures of 20, 23, 26 and 29°C. Survival was measured by quartiles (i.e. time to 75%, 50% and 25% survival) and estimated parameters of the Weibull model fitted to the survival distributions. D. gelbus lived as long or significantly (P= 0.05) longer than the other species at all temperatures. D. maidis (the corn leafhopper) had survival times equal to or significantly shorter than D. elimatus (the Mexican corn leafhopper) at all temperatures except 29°C where D. maidis lived the longest. The shape of the survival curves did not vary among species or change with temperature. The fecundity of D. gelbus, as measured by the average number of eggs laid per female per generation, was equal to or significantly lower than the other species at all temperatures. D. maidis and D. elimatus had similar fecundity at all temperatures except 29°C, where D. maidis produced significantly more eggs. The mean development time from egg to adult declined with temperature between 17 and 29°C. At all temperatures, D. maidis developed the fastest, D. gelbus the slowest, and D. elimatus was intermediate. The results can be explained on the basis of the geographic distribution, plant host species, and life-history strategies of the leafhoppers. Models for describing the population dynamics of leafhoppers are evaluated and discussed.     

Inoculation and acquisition of maize bushy stunt mycoplasma by its leafhopper vector Dalbulus maidis

Maize bushy stunt mycoplasma (MBSM), a mycoplasma-like organism, is transmitted in a persistent manner by the corn leafhopper, Dalbulus maidis, to maize (Zea mays). The influence of the duration of acquisition access and inoculation access periods on the transmission of MBSM by D. maidis was investigated. The proportion of plants infected by D. maidis increased significantly from 0 to 0.51 as the inoculation access time to a plant increased from 10 min to 72 h (X²= 101.5, P < 0.001). Likewise, the proportion of insects acquiring MBSM from infected plants increased from 0 to 0.19 as the acquisition access time to the source plant increased from 10 min to 72 h (X²= 53.2, P < 0.001). The data were fitted to a loglinear regression model. No significant association was found between the sex of the insects and vector ability.     

Oviposition behavior of Coccidoxenoides peregrinus, a parasitoid of Planococcus ficus

We tested hypotheses concerning the specificity of interactions between insect vectors and mollicute plant pathogens in a 22-month study of leafhoppers collected at three agricultural field sites in Mexico. The common species collected, Dalbulus maidis, D. elimatus, D. gelbus, and D. guevari were equally likely to test positive for corn stunt spiroplasma (CSS) in ELISA, and to transmit maize bushy stunt phytoplasma (MBSP) to test maize seedlings. We documented intraspecific variation in the ability of D. maidis to transmit confirmed CSS infections. Dalbulus guevari and D. gelbus were less successful in transmitting CSS than D. maidis from the same population. Our results suggest this vector-plant pathogen interaction is not specific to a single Dalbulus-mollicute combination, and that both the range of potential vectors in agricultural fields, and intraspecific variation across populations of these vectors, should be the focus of future work.     

The parasitoid Gonatopus bartletti reduces presence of plant-pathogenic Spiroplasma kunkelii within the leafhopper vector Dalbulus maidis

Homopteran vectors (e.g., leafhoppers) of plant pathogens are vessels for reproduction of cell wall‐free bacteria. These vectors also serve as hosts for larval parasitoid dipterans, hymenopterans, and strepsipterans. However, no study has explored the relationship among these wall‐free bacteria and parasitoid larvae within the insect host. We studied the corn stunt spiroplasma (CSS), Spiroplasma kunkelii Whitcomb (Mycoplasmatales: Spiroplasmataceae), a bacterium that originated from secondary symbionts that cause corn stunt disease in maize, Zea mays L., and its reproduction in the haemolymph of the corn leafhopper, Dalbulus maidis (Delong and Wolcott) (Homoptera: Cicadellidae). We also studied the dryinid parasitoid Gonatopus bartletti Olmi (Hymenoptera: Dryinidae), the larva of which feeds in the corn leafhopper haemolymph. Our results showed that when CSS and the wasp coexisted in D. maidis, the development of the parasitoid was not affected by S. kunkelii. Parasitoid development was successfully completed when leafhoppers acquired S. kunkelii before or after parasitism and when CSS had median (10 days) and long (20 days) incubation periods in the leafhopper before parasitization. The presence of S. kunkelii did not affect parasitoid development to the adult stage. However, polymerase chain reaction showed that the presence (survival) of S. kunkelii in the leafhopper was negatively affected by the parasitoid larva. Fewer leafhoppers had CSS before and after parasitization compared with leafhoppers that only acquired the CSS. This negative effect helps to explain the high parasitism rate by G. bartletti in D. maidis and the low presence of S. kunkelii in the corn leafhopper when CSS and the wasp parasitoid overlap throughout their geographic distribution. The parasitoid larva may negatively affect S. kunkelii by (1) producing antibacterial peptides that are toxic to CSS; (2) producing teratocytes that take nutrients from the host for larval development, but these nutrients are required by CSS; (3) affecting, indirectly, CSS through other symbiotic microorganisms; and (4) producing proteins with antibacterial activity that are present in the venom of the wasp parasitoid.     

Survival in Dalbulus leafhopper vectors improves after exposure to maize stunting pathogens

Using D. maidis and pathogen isolates collected at three field sites along an altitudinal gradient in Mexico, we compared survival in leafhoppers exposed to healthy maize to those exposed to maize infected with one of four isolates of maize stunting pathogens: two isolates of the corn stunt spiroplasma (CSS, Spiroplasma kunkelii) and two of the maize busby stunt phytoplasma (MBSP). Survival improved after exposure to either plant pathogen under both the cooler and warmer environmental conditions D. maidis is likely to encounter during the dry season. Survival varied among leafhoppers from the different field sites, suggesting that gene flow between these populations is limited. The leafhoppers responded differently to the four isolates (i.e., we noted significant population by exposure interactions), but we found no difference between MBSP and CSS exposure. Finally, we found evidence of local adaptation in one leafhopper population to sympatric, as compared to allopatric, plant pathogens. We have shown with this and our earlier study that aspects of the interaction phenotype in the association between D. maidis and the plant pathogens are mutualistic and that this association has considerable potential as a model for studies of local adaptation.     

Wind tunnel assays of olfactory responses of femaleRhagoletis pomonella flies to apple volatiles: effect of wind speed and odour release rate

Little is known about how adults of the corn leafhopper,Dalbulus maidis (DeLong & Wolcott), and its congeners survive subfreezing temperatures at high elevations during the dry winter in Mexico. In the laboratory, duration of survival at −5°C was measured for four MexicanDalbulus species:D. maidis, D. elimatus (Ball),D. gelbus DeLong andD. quinquenotatus DeLong & Nault; and a closely related North American species,Baldulus tripsaci Kramer & Whitcomb. Adult leafhoppers reared under environmental conditions that simulated the beginning of the dry winter season during October in Mexico (‘October-reared’) were at least twice as tolerant of −5°C than adults reared under environmental conditions that simulated the beginning of the wet summer season during June (‘June-reared’).Dalbulus species found primarily at high elevations, such asD. elimatus, were seven times more tolerant of −5°C thanD. quinquenotatus, a species which overwinters at low to mid elevations on itsTripsacum hosts. October-rearedD. maidis adults survived relatively short periods at −5°C (LT₅₀=8.9h) compared to October-rearedD. elimatus adults (LT₅₀=42.3h). This suggests that in Mexico,D. maidis either overwinters in protected habitats at higher elevations or it migrates to lower, frost-free regions. October-rearedB. tripsaci adults, which overwinter in the egg stage, were intolerant of −5°C (LT₅₀=2.6h). A conditioning period for 1 h at +5°C before and after exposure to −5°C significantly improved survival forD. maidis. Supercooling points (SCPs) were between −23 and −20°C, indicating that mortality of these leafhoppers at −5°C was due to cold shock injury rather than internal ice formation.     

Field overwintering biology of Spiroplasma kunkelii (Mycoplasmatales: Spiroplasmataceae) and its vector Dalbulus maidis (Hemiptera: Cicadellidae)

We studied the corn stunt spiroplasma (CSS), Spiroplasma kunkelii (Mycoplasmatales: Spiroplasmataceae) and its vector the corn leafhopper Dalbulus maidis (Hemiptera: Cicadellidae) under field conditions in Mexico. We surveyed for the presence of CSS in D. maidis by using PCR on samples of adults collected during the 2000–01 and 2003–04 winter (dry) seasons from irrigated low‐elevation sites and un‐irrigated high‐elevation sites. Also, we determined the body size and number of mature eggs of D. maidis females collected during the dry season in 2004 and in females collected on maize seedlings in the first months (June and July) of the wet (summer) season in 2005. Our PCR results showed that CSS was present in leafhopper adults collected during the 2000–01 and 2003–04 dry seasons in irrigated low‐elevation sites. However, in un‐irrigated high‐elevation sites, CSS was present in corn leafhopper adults caught before, but not during, the dry seasons. In these un‐irrigated high‐elevation sites, adult leafhoppers without CSS were recovered during the first 2 months (November and December) of the dry season from the foliage of wild perennial grasses. Females collected on wild perennial grasses in December 2004 showed similar head width and wing length to females caught on maize seedlings in June 2005. However, females collected on maize seedlings in July 2005 had the widest heads, longest wings and highest number of mature eggs. The largest body size of these females that arrived in July 2005 indicates that they are immigrants and support the Roff’s hypothesis that insect migrants should be larger than nonmigrants.     

Survival,fecundity, and body mass of Amplicephalus curtulus influenced by ‘Candidatus Phytoplasma ulmi’ (16SrV‐A) infection

The leafhopper Amplicephalus curtulus Linnavuori & DeLong (Hemiptera: Cicadellidae) can transmit ‘Candidatus Phytoplasma ulmi’ (16SrV‐A) from a native Chilean shrub, Ugni molinae Turcz. (Myrtaceae), to ryegrasses. A recent study showed that this phytoplasma reduced the total protein content and the activity of detoxifying enzymes in A. curtulus, which could also affect its vector fitness. This study evaluated the effect of ‘Ca. Phytoplasma ulmi’ on the longevity, fecundity, and body mass of A. curtulus. Both females and males were exposed to ‘Ca. Phytoplasma ulmi’‐infected plants for 96 h, whereas a control group remained unexposed. Quartiles from adult emergence to 75% (t₇₅), 50% (t₅₀), and 25% (t₂₅) survival rates were determined for each leafhopper survival distribution. The dry weight was also established at the end of the experiment. The adult lifespan of phytoplasma‐infected males and females was significantly lower than that of the uninfected leafhoppers in quartile survival distributions t₅₀ and t₂₅. The phytoplasma‐infected males and females lived 3 and 4 weeks less than uninfected ones in the last quartile, respectively. Fecundity was established by number of nymphs per female (in four periods) in phytoplasma‐infected and uninfected assays. In general, the weekly pattern of the number of nymphs per phytoplasma‐infected female was lower than that of uninfected leafhoppers; it was 37% lower at the end of the experiment. Phytoplasma‐infected females weighed significantly less (11%) than uninfected individuals. Phytoplasma‐infected males weighed 8% less than uninfected ones, but this difference was not significant. Our data indicated that ‘Ca. Phytoplasma ulmi’ negatively affected the fitness of A. curtulus, and nymphs produced by phytoplasma‐infected females varied over time, which may influence the disease dynamics in nature or in field crops.     

Acquisition and transmission of corn stunt spiroplasma by its leafhopper vector Dalbulus maidis

As the acquisition access period of Dalbulus maidis on infected maize increased from 15 min to 7 days, the incubation period of corn stunt spiroplasma (CSS) in the insect decreased from 27 days to 8 days and the final proportion of transmitting insects increased from 5% to 100%. After 7 days access the median incubation period (IPsO) was 14.3 days (IP₅₀ females = 12.9 days: IP₅₀ males =16.8 days), while the proportion of transmitting insects increased from 4. 3% (9 days after the start of acquisition access) to a maximum of 93% (after 22 days), before decreasing. Females started transmitting significantly earlier and a greater proportion transmitted each day than males, until day 22 when both sexes transmitted equally. Of the insects which transmitted CSS, 29% did so continuously until death; 66% failed to transmit during the last 1–3 days, and 5% transmitted intermittently towards the end of their life. During daily transfer, females were more likely to infect plants consecutively (up to 25) than males, and females infected the higher proportion of test plants. As the transmission access period was increased from 1 h to 72 h, the proportion of transmitting insects increased from 22.5% to 97.3% and the incubation period in maize decreased.     

Management of corn leafhopper (Homoptera: Cicadellidae) and corn stunt disease in sweet corn using reflective mulch

Plastic reflective mulches significantly reduced populations of corn leafhopper, Dalbulus maidis (DeLong & Wolcott), adults and the incidence of corn stunt disease caused by Spiroplasma kunkelii (CSS) in late planted sweet corn (Zea mays L.). The reflective mulches were more effective than were either foliar or soil applied insecticides in managing both the leafhopper and the pathogen it transmits. Yields of marketable ears were 1.5 to 2 times greater in reflective mulch plots than from fallow plots. This was due to larger ears (individual ear weight and length) rather than an increase in the number of ears. The use of reflective mulches provides an alternative strategy to insecticides in the management of both D. maidis and corn stunt disease. Such a strategy may prove useful to growers in Latin America and to limited resource growers and organic growers in the United States who wish to grow corn without the use of insecticides.     

Increased survival of Dalbulus maidis, a specialist on maize, on non-host plants infected with mollicute plant pathogens

The leafhopper Dalbulus maidis DeLong & Wolcott survived significantly longer on aster, Callistephus chinensis Nees, infected with any one of 3 strains of aster yellows (AY) mycoplasma-like organism (MLO) than on healthy asters. After 7 or more days on AY-diseased aster, females were conditioned to survive longer on healthy asters than were leafhoppers of the same age previously exposed only to maize. Females were also conditioned to survive longer on healthy aster by prior exposure to AY-MLO-infected celery (Apium graveolens L.). Males were not so conditioned. Leafhoppers injected with infectious extracts of AY-MLO dit not live longer on aster nor transmit the AY-MLO to aster. Conditioning on AY-diseased aster did not cause D. maidis to transmit AY-MLO and did not interfere with the transmission to maize of the mollicute (Spiroplasma kunkelii Whitcomb et al.) that causes corn stunt disease. Spiroplasma citri Saglio et al. infection of aster but not of turnip (Brassica rapa L.), Plantago major L. or periwinkle (Catharanthus roseus (L.)), improved the longevity of D. maidis on these plants and conditioned leafhoppers for enhanced subsequent survival on healthy asters.

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Résumé La cicadelle du maïs, D. maidis à survécu significativement plus longtemps sur aster (Callistephus chinensis Nees) contaminé par l'une des trois souches de l'agent mycoplasmique (MLO) de la jaunisse de l'aster (AY), que sur des asters sains. Après 7 jours ou plus sur des asters AY-MLO, les femelles (et non les mâles) étaient conditionnées de telle sorte qu'elles survivaient plus longtemps sur asters sains que des femelles du même âge précédemment sur maïs. Sur des céleris (Apium graveolens) atteints de AY, D. maidis a survécu plus longtemps que sur céleri sain, mais moins que sur aster AY. Des extraits, contaminés par AY injectés à D. maidis n'ont pas augmenté sa longévité sur aster, ni provoqué la transmission de AY-MLO. Le conditionnement sur aster AY n'a pas entrainé la transmission de AY-MLO par D. maidis, et n'a pas interferé avec son aptitude à transmettre l'agent du nanisme du maïs, Spiroplasma kunkelii. S. citri a aussi conditionné D. maidis sur aster, mais a été sans effet sur navet (Brassica rapa), sur plantain (Plantago major) et sur pervenche (Vinca).

     

Leafhopper response to genetically diverse maize stands

At a site in Nicaragua with high population densities of the leafhopper Dalbulus maidis Delong & Wolcott, leafhopper densities were significantly lower in mixed stands of maize (Zea mays mays L.) varieties than would be expected by averaging the densities found in the pure stands of the component varieties. This response to genetic diversity appears to be due to a behavioral response during the period of colonization or establishment. The reduction in leafhopper abundance was not clearly reflected in a reduction in the incidence of the corn stunt pathogen transmitted by the leafhopper, probably because of increased leafhopper movement in mixed stands.

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Résumé Deux champs du Nicaragua, très différents quant aux densités moyennes de la cicadelle, Dalbulus maidis, et à la fréquence du nanisme par spiroplasme du maïs, — dont la cicadelle est le vecteur —, ont servi à l'étude de l'influence de l'hétérogénéité génétique sur l'abondance de Dalbulus maidis Delong & Wolcott. La maïs a été semé en parcelles pures d'une seule variété ou en parcelles génétiquement hétérogènes avec mélange de 5 variétés. Dans la zone où D. maidis est très abondant, la densité de la cicadelle était significativement plus faible dans les parcelles hétérogènes que ne le laissaient prévoir les densités moyennes observées dans les parcelles des variétés pures intervenant dans le mélange. Cette réponse à l'hétérogénéité génétique semble due aux réactions comportementales pendant la période de colonisation ou d'installation. La diminution de l'abondance de D. maidis ne se traduit pas nettement par une réduction de la fréquence du nanisme du maïs, peutêtre par suite d'un accroissement des mouvements de cicadelles dans les parcelles hétérogènes.

     

Isolation and Culture of Corn Stunt Spiroplasma in Serum-Free Medium

A serum-free medium (medium A₆), supports the primary isolation and cultivation in vitro of corn stunt spiroplasma (CSS). The growth characteristics of CSS in A₆ medium are similar to those in the conventional serum dependent, ketoglutarate sorbitol medium (SMK).     

Effects of temperature on the development,performance and fitness of the corn leafhopper Dalbulus maidis (DeLong) (Hemiptera: Cicadellidae): implications on its distribution under climate change

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Influence of adult diet on fitness and reproductive traits of the egg parasitoid Anagrus virlai (Hymenoptera: Mymaridae), a potential biocontrol agent against the corn leafhopper

Dalbulus maidis is the most important leafhopper pest of maize in the Americas. Anagrus virlai is an egg parasitoid commonly associated with the corn leafhopper. We evaluated whether the performance of A. virlai is dependent on different diets provided during 24 hr or throughout adult female lifetime. Additionally, functional response of A. virlai on D. maidis eggs using maize leaves containing honeydew plus honey was described. A. virlai is a mostly pro-ovigenic autogenic species whose females are able to parasitize eggs immediately after emergence. We found that wasps oviposit the majority of their eggs in the first day of their adult life. Realized lifetime fecundity and lifetime potential fecundity did not differ significantly among treatments, but longevity and egg production increased when honey was added to diet. Data were consistent with an intermediate functional response between type II and III, but closer to type II, indicating a high parasitism rate at low host densities and a decrease in the oviposition rate at high host densities, due to a possible egg limitation. Our results suggest that carbohydrate food sources (honeydew and honey) might not be the factor limiting reproductive success during the first 24 hr. Food supply, however, might influence egg maturation and survivorship of wasps, thus potentially enhancing biological pest control when hosts are scarce in the course of the first few days of adulthood.     

Do assortative mating and immigrant inviability help maintain population genetic structuring of an herbivore on a crop and a wild relative?

Population genetic structuring is common among herbivorous insects and frequently is associated with divergent host plants, such as crops and their wild relatives. Previous studies showed population genetic structuring in corn leafhopper Dulbulus maidis in Mexico, such that the species consists of two sympatric, host plant-associated populations: an abundant and widespread "pestiferous” population on maize (Zea mays mays), and a small and localized "wild" population on perennial teosinte (Zea diploperennis). a maize wild relative with a limited distribution. This study addressed whether assortative mating and immigrant inviability mediate genetic structuring of corn leafliopper by comparing the mating and reproductive successes of pestiferous and wild females that colonize their nonassociated host plants against the successes of females colonizing their associated host plants. Assortative mating was assessed by comparing mating frequencies and premating and mating times among females of each population on each host plant: immigrant inviability was assessed by comparing, across two generations, the fecundity, survival, development time, sex ratio, and population growth rate among leafhopper populations and host plants. Our results showed that on maize, and compared to resident, pestiferous females, wild females were more likely to mate, and greater proportions of their offspring survived to adult stage and were daughters;consequently, the per-generation population growth rate on maize was greater for immigrant, wild leafhoppers compared to resident, pestiferous leafhoppers. Our results suggested that wild leafhoppers emigrating to maize have a fitness advantage over resident, pestiferous leafhoppers, while immigrant pestiferous and resident wild leafhoppers on teosinte have similar fitnesses.     

Transmission of rayado fino virus of maize (Zea mays) by Dalbulus tnaidis

Rayado fino virus (RFV) of maize (Zea mays) was transmitted by the leaf-hopper Dalbulus maidis in a manner characteristic of viruses that multiply in their insect vectors. Individual insects fed on infected plants transmitted the virus after incubation periods of 8–22 days; males had shorter incubation periods than females but died sooner. Insects retained infectivity for 1–20 days. Transmission by most insects was intermittent. Inoculativity by D. maidis decreased with time, but the virus was recovered from insects that had lost their ability to transmit. Extracts of plants infected with RFV and viruliferous insects were injected into healthy insects, which became viruli-ferous. Infectivity of the extracts was not affected by tetracycline hydrochloride (Achromycin). D. maidis was able to transmit simultaneously RFV and the corn stunt agent. Other than maize, Teosinte (Euchlaena mexicana) was the only plant susceptible to the virus, among a number of species of Gramineae tested.     

Corn Stunt Spiroplasma in Dicotyledonous Plants

Corn stunt spiroplasma (CSS) was transmitted by the leafhopper vector Euscelidius variegatus (Kirschbaum) and produced symptoms on four dicotyledonous plant species, Sinapis alba L. (mustard), Pisum sativitm L. (pea), Raphanus sativus L. (radish) and Spinacia oleracea L. (spinach). The vectors became infective by microinjection with a broth culture of CSS. This insect also acquired CSS from infected mustard plants and transmitted it to healthy ones.     

Reduced Mythimna separata infestation on Bt corn could benefit aphids

Use of genetically engineered plants that express insecticidal Cry proteins derived from Bacillus thuringiensis (Bt) have been proven efficacious for managing lepidopteran pests. However, in some cases herbivores that are not targeted by the Bt trait have increased in importance. It has been suggested that reduced caterpillar damage to Bt crops could lead to decreased levels of induced plant defensive compounds which might benefit other non-target herbivores. Here we investigated the potential effect of reduced damage by larvae of Mythimna separata on aphid populations in Bt corn. We compared the performance of Rhopalosiphum maidis feeding on non-Bt corn plants that had been infested by M. separata larvae or were uninfested. The results showed that caterpillar-infested corn plants significantly reduced the fitness of R. maidis leading to a prolonged nymphal development time, reduced adult longevity and fecundity compared to uninfested plants. Consequently, the population growth rate of corn aphids feeding on caterpillar-infested corn plants was significantly lower than on uninfested plants. As expected, the aphids performed significantly better on Lepidoptera-resistant Bt corn than on non-Bt corn when plants were infested with M. separata, since the caterpillars caused very little damage to the Bt plants. The current findings indicate that reduced M. separata infestation could benefit aphid development in Bt corn. Bt corn has the potential to be commercialized in China in the near future and aphids and other non-target pests should be monitored in the farming fields.     

Facilitated by nature and agriculture: performance of a specialist herbivore improves with host-plant life history evolution, domestication, and breeding

Plant defenses against herbivores are predicted to change as plant lineages diversify, and with domestication and subsequent selection and breeding in the case of crop plants. We addressed whether defense against a specialist herbivore declined coincidently with life history evolution, domestication, and breeding within the grass genus Zea (Poaceae). For this, we assessed performance of corn leafhopper (Dalbulus maidis) following colonization of one of four Zea species containing three successive transitions: the evolutionary transition from perennial to annual life cycle, the agricultural transition from wild annual grass to primitive crop cultivar, and the agronomic transition from primitive to modern crop cultivar. Performance of corn leafhopper was measured through seven variables relevant to development speed, survivorship, fecundity, and body size. The plants included in our study were perennial teosinte (Zea diploperennis), Balsas teosinte (Zea mays parviglumis), a landrace maize (Zea mays mays), and a hybrid maize. Perennial teosinte is a perennial, iteroparous species, and is basal in Zea; Balsas teosinte is an annual species, and the progenitor of maize; the landrace maize is a primitive, genetically diverse cultivar, and is ancestral to the hybrid maize; and, the hybrid maize is a highly inbred, modern cultivar. Performance of corn leafhopper was poorest on perennial teosinte, intermediate on Balsas teosinte and landrace maize, and best on hybrid maize, consistent with our expectation of declining defense from perennial teosinte to hybrid maize. Overall, our results indicated that corn leafhopper performance increased most with the agronomic transition, followed by the life history transition, and least with the domestication transition.