Aphid Population Growth Research Articles

Effects of sugarcane aphid herbivory on transcriptional responses of resistant and susceptible sorghum

BackgroundSugarcane aphid (Melanaphis sacchari) outbreaks in sorghum that were first reported in 2013 are now the most significant threat to this crop in all major sorghum production areas in the U.S. The outcomes of interactions between sugarcane aphid and sorghum and thus the severity of the outbreaks depend on sorghum genotype and potentially also on the phenology of sorghum. Mechanisms underlying these interactions are not known, however. Thus, the goal of this research was to characterize transcriptional changes in a commercially available resistant and a susceptible genotype of sorghum at 2- and 6-wk post-emergence exposed to M. sacchari herbivory. The effects of sorghum age and genotype on the daily change in aphid densities were also evaluated in separate greenhouse experiments.ResultsA higher number of diffentially expressed genes (DEGs) was recovered from the 2-wk plants exposed to aphid herbivory compared to the 6-wk plants across genotypes. Further, gene ontology and pathway analysis indicated a suite of transcriptional changes in the resistant genotype that were weak or absent in the susceptible sorghum. Specifically, the aphid-resistant genotype exposed to M. sacchari up-regulated several genes involved in defense, which was particularly evident in the 2-wk plants that showed the most robust transcriptional responses. These transcriptional changes in the younger resistant sorghum were characterized by induction of hormone-signaling pathways, pathways coding for secondary metabolites, glutathion metabolism, and plant-pathogen interaction. Furthermore, the 2-wk resistant plants appeared to compensate for the effects of oxidative stress induced by sugarcane aphid herbivory with elevated expression of genes involved in detoxification. These transcriptional responses were reflected in the aphid population growth, which was significantly faster in the susceptible and older sorghum than in the resistant and younger plants.ConclusionThis experiment provided the first insights into molecular mechanisms underlying lower population growth of M. sacchari on the resistant sorghum genotype. Further, it appears that the younger resistant sorghum was able to mount a robust defense response following aphid herbivory, which was much weaker in the older sorghum. Several pathways and specific genes provide specific clues into the mechanisms underlying host plant resistance to this invasive insect.

The effects of sugarcane aphid density in sorghum on predation by lady beetles and lacewings

The sugarcane aphid, Melanaphis sacchari Zehntner (Hemiptera: Aphididae), is a new and devastating pest of sorghum (Sorghum bicolor (L.)) in the United States. Arthropod predators, especially lady beetles and green lacewings, are often abundant in sugarcane aphid-infested sorghum, but little is known about the effects of these predators on sugarcane aphid populations. We quantified the effect of two species of lady beetles (larvae and adults of Coccinella septempunctata L. and Harmonia axyridis Pallas) (Coleoptera: Coccinellidae) and green lacewings (larvae of Chrysoperla rufilabris Burmeister) (Neuroptera: Chrysopidae) on aphid colony growth across several aphid densities (20, 40, 80, 160). We found that all predator species and life stages significantly reduced the growth of sugarcane aphid populations at low to intermediate aphid densities (20, 40, and 80 aphids per colony). At the highest aphid density (160 aphids), however, H. axyridis larvae were much less effective than C. septempunctata larvae and adults and H. axyridis adults and lacewing larvae did not suppress sugarcane aphid colony growth. We incorporated these predator effects into a dynamic predator-prey model to predict the impact of predators on the short-term population dynamics of sugarcane aphids in sorghum (2–14 days) and the ability of these predators to suppress aphids below action thresholds (50–75 aphids per leaf). The model illustrated that all the predators we studied prevented aphid densities between 20 and 40 per leaf from reaching action thresholds. At initial densities of 80 aphids per colony, all predators were able to suppress aphids below threshold in two to four days. At the highest initial aphid density (160), however, H. axyridis larvae could not suppress aphids below the action threshold by four days and lacewing larvae never suppressed aphids below the threshold. Our results suggest that common predators in sorghum have the potential to suppress sugarcane aphid population growth and reduce the number of insecticide applications needed to control sugarcane aphids.

Landscape structure alters the abundance and species composition of early-season aphid populations in wheat fields

Agricultural landscape composition is reported to have effects on the occurrence of natural enemies in fields, but the responses of herbivorous pests to landscape features have rarely been studied. Our previous landscape-level study in northern China found that natural enemies had no significant effects on the population growth of wheat aphids at the colonization stage. Hence, we hypothesised that the initial aphid population in wheat fields may largely depend on immigration events from other habitats. In this study, we investigated the effects of landscape pattern on wheat aphid abundance and species composition at the colonization stage by surveying population densities of different aphid species along a landscape gradient. We found that noncrop habitats such as woodlots, fallow lands and vegetation around dwellings or wetlands in the landscape all had positive correlations with aphid abundance in wheat fields because they provided a source for aphid colonization. More specifically, the proportion of woodlots, fallow lands, dwellings, and other crops increased the abundance of Rhopalosiphum padi (L.), and the existence of water and dwellings in an agricultural landscape facilitated the occurrence of Sitobion avenae (Fabricius). Likewise, the abundance of Schizaphis graminum (Rondani) in wheat fields was found to be significantly promoted by the presence of water. The habitats around water and dwellings usually provide the overwintering sites for the aphids. Furthermore, wheat production acreage had a significant negative correlation with the abundance of R. padi, S. avenae and S. graminum. Landscape characteristics had a significant effect on the species composition of wheat aphids. Our study suggests that noncrop habitats in a landscape enhance aphid occurrence at the population colonization stage, and differences in overwintering host plants among aphid species may be responsible for the differing responses of the abundance of different aphids to landscape variables, which greatly altered aphid population composition across different landscapes. These results highlight the need for taking landscape effects on the pest itself into consideration when designing landscape-level pest management strategies.

Asymmetric effects of a leaf‐chewing herbivore on aphid population growth

1. Plant responses to herbivory are often specific to the feeding guild of the attacking herbivore. These phytochemical responses to herbivore damage can affect herbivore performance and activity. Comprehensive studies on the ecological consequences of multi‐herbivore plant interactions are key to understanding plant–herbivore community dynamics.2. This study examined how feeding damage by co‐occurring herbivores from separate feeding guilds, Myzus persicae (Sulzer), a sucking herbivore, and Leptinotarsa decemlineata (Say), a chewing herbivore, alter plant chemistry and indirectly affect herbivore performance. Performance was measured when each insect fed on plants individually, sequentially, or simultaneously in laboratory and field experiments. Phytohormone and glycoalkaloid content were measured for each feeding sequence to evaluate plant responses to herbivory by each guild. Mid‐season and end‐of‐season tuber yield were evaluated in the field study.3. Damage by L. decemlineata negatively impacted M. persicae performance in both laboratory and field settings. Damage by M. persicae did not affect L. decemlineata performance in laboratory assays. However, L. decemlineata performance was positively affected by M. persicae herbivory in the field, but this effect was temporary. Although phytohormones and plant defences varied across treatments, they provide little resolution on interaction outcomes.4. These results confirm that the presence of multiple feeding guilds on a single plant can affect these chewing and sucking herbivores differentially, but given the variability in our phytochemical analyses compared with other studies, the mechanism remains unclear. The study's findings show that aphids are negatively affected by chewing herbivores across systems, while aphids temporarily affected beetles positively.

Impact of the parasitoid Aphelinus certus on soybean aphid populations

Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae) is an accidentally introduced parasitoid of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) in North America and it has become one of the most common natural enemies of soybean aphids in its adventive range. It is unclear, however, if increased prevalence of A. certus has resulted in increased biological control. We conducted an exclusion-cage experiment designed to isolate the impact of parasitoids from that of other resident natural enemies (mainly predators) of the soybean aphid. We found that A. certus greatly outnumbered all other soybean aphid parasitoids, and that it significantly reduced soybean aphid populations over a time span of less than two weeks compared to controls. Moreover, parasitoids alone resulted in aphid densities that were statistically equivalent to the combined effect of predators and parasitoids. Across all treatment cages, there was a significant negative association between parasitism rate and aphid population growth, with predicted zero aphid growth occurring at a parasitism rate of 42.2%.

Key aphid natural enemies showing positive effects on wheat yield through biocontrol services in northern China

Aphids constitute a group of major wheat crop pests in northern China and severe infestations at heading and flowering stages lead to important yield loss. Natural enemies may increase yield by reducing aphid density but alteration in cropping pattern can cause changes in their species composition. We observed strong evidence for top-down control of the wheat aphid population growth in winter wheat during 2011-2013 at Langfang Experimental Station (CAAS, China), e.g., a significant increase in the control efficiency of natural enemies in open field cages accounted for 98, 53.8 and 59.3% in the 2011, 2012 and 2013, respectively as compared to exclusion cages. We observed peak population of wheat aphids in the middle growth stage in open field survey. Overall, the population densities of wheat aphids and natural enemies were highest in the middle and late growth stages, respectively while lowest densities recorded in late stage for aphids and in early plant stage for natural enemies. The potential natural enemies were coccinellids, and Aphidiinae aphid parasitoids, especially Aphidius gifuensis (Ashmead) following the aphid peaks each season. Yield difference among the three exclosure cages were substantially different during 2011–2012, the highest yield was recorded in the treatment without aphids as compared the others. Compared to previous studies conducted in northern China, the control efficiency of natural enemies has been greatly increased by coccinellids and parasitoid community in the absence of chrysopids and spiders. These biocontrol services will provide improved IPM strategies for the wheat ecosystem in northern China.

Aphid infestation leads to plant part-specific changes in phloem sap chemistry, which may indicate niche construction.

Phloem sap quality can differ between and within plants, and affect the performance of aphids. In turn, aphid infestation may change the chemical composition and nutritional value of phloem sap. However, the effects of different aphid species on the overall phloem sap composition of distinct parts within plant individuals in relation to aphid performance remain unclear. To test the specificity of plant responses to aphids, we used two chemotypes of Tanacetum vulgare plants and placed the monophagous aphids Macrosiphoniella tanacetaria and Uroleucon tanaceti on different plant parts (stems close to the inflorescence, young and old leaves). Aphid population growth was determined and sugars, organic acids, amino acids and metabolic fingerprints of phloem exudates were analysed. Macrosiphoniella tanacetaria performed best on stems, whereas U.tanaceti performed best on old leaves, indicating differences in niche conformance. Aphid infestation led to distinct changes in the phloem exudate composition of distinct metabolite classes, differing particularly between plant parts but less between chemotypes. In summary, plant responses to aphids are highly specific for the chemotype, plant part, metabolite class and aphid species. These changes may indicate that aphids construct their own niche, optimizing the food quality on the plant parts they prefer.

Silicon-mediated and constitutive resistance to Rhopalosiphum maidis (Hemiptera: Aphididae) in corn hybrids.

The corn leaf aphid, Rhopalosiphum maidis (Fitch) (Hemiptera: Aphididae), is an important pest of corn, but no corn genotypes resistant to R. maidis are commercially available. Although the ability of silicon to induce plant resistance against some insects is known, the effect of silicon on R. maidis and in corn hybrids with different levels of constitutive resistance is still unknown. This study sought to determine the constitutive resistance of corn hybrids to R. maidis and silicon resistance induction in hybrids with different degrees of constitutive resistance. Field experiments with natural infestations of aphids were conducted in three locations in Brazil (Patos de Minas, Araguari, and Tupaciguara). Greenhouse trials were also used to evaluate the effect of varietal resistance on aphid population growth and identify resistant and susceptible genotypes. Aphid resistance induced by silicon was determined with resistant and susceptible corn hybrids. In the field, the corn hybrids BM8850, AS1625PRO, and DKB310PRO had the greatest proportion of plants infested by R. maidis in all three localities. The hybrids P30F53H, STATUS VIP, BM9288, DAS2B587HX, DKB175PRO, AS1633PRO, and DKB390PRO2 were the least infested in Patos de Minas and Araguari, and P30F53H was the least infested in Tupaciguara. When antibiosis effects were evaluated by aphid population growth, the hybrids AG7088PRO3 and DKB310PRO2 were susceptible, while P30F53YH was resistant. When natural aphid infestation was evaluated, wherein the effects of antibiosis and non-preference could not be discriminated, soil applications of silicon-induced resistance to R. maidis in both susceptible and constitutively resistant corn hybrids.

Differences in Hormonal Signaling Triggered by Two Root-Feeding Nematode Species Result in Contrasting Effects on Aphid Population Growth

NvD gratefully acknowledges the support of the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig funded by the German Research Foundation (FZT 118). MW was supported by an Erasmus Mundus fellowship for the European Master of Science in Nematology (EUMAINE).

What doesn’t kill you makes you stronger: The burdens and benefits of toxin sequestration in a milkweed aphid

Abstract Specialized insect herbivores commonly co‐opt plant defences for protection against predators, but the costs, benefits and mechanisms of sequestration are poorly understood. We quantified sequestration of toxic cardenolides by the specialist aphid Aphis nerii when reared on four closely related milkweed (Asclepias) species with >20‐fold variation in cardenolide content, and in the presence or absence of generalist ladybug predators. Increasing concentrations of apolar plant cardenolides increased sequestered amounts in aphids. High concentrations in plants impaired aphid population growth, but also reduced the top‐down effects of predators. An in vitro enzymatic assay of total cardenolides in aphid bodies using the cardenolides’ target (animal Na+/K+‐ATPase) revealed that the subset of sequestered cardenolides is disproportionally more toxic than cardenolides in leaves. All aphids accumulated two cardenolides not present in their host plant, even on plants with very low foliar cardenolide concentrations. Sequestration of potent cardenolides by A. nerii thus involves passive, concentration‐dependent uptake from the host plant, as well as a presumably more active mechanism of modification and up‐concentration of plant cardenolides. The concentration of toxins in the host plant thus not only determines the negative impacts on growth and performance of an aphid, but also the ease and efficiency by which toxins are sequestered for the aphid’s defence, making the costs and benefits of plant toxins highly context‐dependent for both the plant and the herbivore. Therefore, variation in plant toxins is of central importance for co‐evolutionary plant–insect interactions, particularly in environments with variable predator pressure. A plain language summary is available for this article.

Plant oxidative status under ozone pollution as predictor for aphid population growth: The case of Metopolophium dirhodum (Hemiptera: Aphididae) in Triticum aestivum (Poales: Poaceae)

Ozone is a secondary air pollutant that affects plants and animals through several physiological mechanisms that involve changes in redox status. However, the consequences of ozone pollution on aphids are not well understood. Therefore, we have experimentally tested: if oxidative stress on the host plant affects lipid peroxidation in aphids or aphid population growth. Wheat plants (Triticum aestivum) were exposed to 140 p.p.b. of ozone or filtered air in open top chambers for three consecutive days and Metopolophium dirhodum (Walker, 1849, Hemiptera: Aphididae) aphids were transferred to the plants immediately after ozone exposure or 72 h later. Ozone exposure reduced antioxidant potential within plant tissues and had no effect on plants’ lipid peroxidation. Lipid peroxidation in aphids fed upon these plants was similar among treatments. Although aphids successfully colonised the plants in all the treatments, the populations established on plants immediately after ozone exposure grew at higher rates than those established 72 h after ozone exposure had ended, independently of ozone level. In conclusion, aphids were tolerant to plant mediated effects of ozone. Therefore, a greater attention should be put in the direct effects of ozone on M. dirhodum - T. aestivum interaction.

Perennial fallow strips support biological pest control in spring cereal in Northern Europe

Fallow strips are promoted through agri-climate-environment schemes to enhance farmland biodiversity, but their effects on biodiversity-mediated ecosystem services remain poorly studied in different cropping systems. The effects of fallow strips on biological control of bird cherry-oat aphids in spring barley were examined in a field experiment in Finland in 2014–2016. Two types of sown perennial fallow strips – grass strip and wildflower strip – were compared. The relative efficiency of biological pest control was tested by monitoring the number of experimentally added aphids in the fallow strips and in the adjacent barley fields at different distances from the strips. Data were also collected on the densities of naturally occurring aphids and ground-dwelling natural enemies. The effects of the fallow strips differed between years. When aphid pressure was high, the proximity of the strips slowed the growth of aphid population. In the year of low aphid abundance, the fallow strips sustained more aphids than the adjacent barley fields, but did not constitute a risk for pest control. In the second and third years, especially wildflower strips supported high activity density of ground-dwelling predators. The activity densities were particularly high in the both fallow strip types in early summer, highlighting the role of the perennial strips as refuges for natural enemies. The results suggest that perennial fallow strips, and especially those sown with diverse wildflower mixtures, can promote pest control in spring cereals, thus contributing to ecological intensification in agriculture.

Change in ladybeetle abundance and biological control of wheat aphids over time in agricultural landscape

The improvement of biocontrol services provided by natural enemies is important for realizing sustainable agriculture. While it has been well demonstrated that landscape context affects the occurrence of natural enemies in agricultural ecosystems, the response of natural enemies and their ability to suppress pest population growth over time has rarely been studied in relation to landscape composition and diversity. In this study, we investigated natural enemy (ladybeetle) and pest (wheat aphids) population abundance in wheat fields in the early period (defined as the main period for aphid colonization) and the late period (i.e., the main period for aphid reproduction) in different landscapes. Non-crop habitats surrounding wheat fields were found to enhance ladybeetle abundance. More specifically, the proportion of dwellings and woodlots were positively correlated with ladybeetle abundance in early period, and those of fallow and woodlot habitats were positively associated with ladybeetle abundance in late period. The most predictive spatial scales of ladybeetle abundance were landscape features within 0.5 and 2.0 km of wheat fields in the early and late periods, respectively. In contrast with the significantly negative correlation between ladybeetle abundance and aphid population growth during the late period, the relative low density of ladybeetles in the early period appeared to have no effect on aphid population growth. These findings suggest that non-crop habitats have differing effects on ladybeetle populations, both in their spatial scale and over time, and the continuity of functional habitat types is essential to support ladybeetle populations. While ladybeetles do indeed suppress aphid growth in the later part of the cycle, aphid control is also dependent on early colonization and constrained by external circumstances.

Predators and alate immigration influence the season-long dynamics of soybean aphid (Hemiptera: Aphididae)

The identification of key species within multi-predator assemblages suppressing agricultural pests is paramount to the incorporation of predators into pest management plans. Generalist predators have strong impacts on population growth of soybean aphids (Aphis glycines, Hemiptera: Aphididae) in North America, but their relative influence may vary seasonally, and potentially interact with immigration of alate aphids following initial field colonization. Here we present the results of a season-long study in Minnesota that used cage manipulations to estimate weekly impacts of predation on field populations of soybean aphids, while concurrently quantifying alate aphid and predator densities. We used generalized linear models based on field level aphid and predator counts, and aphid counts on caged and uncaged experimental plants, to show that Harmonia axyridis (Coleoptera: Coccinellidae) was negatively associated with A. glycines abundance, even under alate immigration levels associated with aphid outbreaks, suggesting potential regulation of aphid populations. In contrast, two other common predators, Coccinella septempunctata (Coleoptera: Coccinellidae) and Orius insidiosus (Hemiptera: Anthocoridae), did not show patterns of association with aphid dynamics. We also show that A. glycines populations only have significant positive associations with the number of immigrating alatae on uncaged experimental plants that had higher alate densities than the field average. The negative effect of H. axyridis on aphid populations was also observed on uncaged experimental plants, suggesting that even high levels of alate aphid immigration did not disrupt predator suppression. These results add to the growing body of evidence demonstrating that even low numbers of certain generalist predators can effectively suppress pest populations in agroecosystems. In particular, our findings suggest that the abundance of both H. axyridis and alate aphids are critical to soybean aphid seasonal dynamics.

Impact of water-deficit stress on tritrophic interactions in a wheat-aphid-parasitoid system.

Increasing temperature and CO2 concentrations can alter tritrophic interactions in ecosystems, but the impact of increasingly severe drought on such interactions is not well understood. We examined the response of a wheat-aphid-parasitoid system to variation in water-deficit stress levels. Our results showed that arid area clones of the aphid, Sitobion avenae (Fabricius), tended to have longer developmental times compared to semiarid and moist area clones, and the development of S. avenae clones tended to be slower with increasing levels of water-deficit. Body sizes of S. avenae clones from all areas decreased with increasing water-deficit levels, indicating their declining adaptation potential under drought. Compared to arid area clones, moist area clones of S. avenae had a higher frequency of backing under severe water stress only, but a higher frequency of kicking under well-watered conditions only, suggesting a water-deficit level dependent pattern of resistance against the parasitoid, Aphidius gifuensis (Ashmead). The number of S. avenae individuals attacked by the parasitoid in 10 min showed a tendency to decrease with increasing water-deficit levels. Clones of S. avenae tended to have lower parasitism rates under treatments with higher water-deficit levels. The development of the parasitoid tended to be slower under higher levels of water-deficit stress. Thus, the bottom-up effects of water-deficit stressed plants were negative on S. avenae. However, the top-down effects via parasitoids were compromised by water-deficit, which could favor the growth of aphid populations. Overall, the first trophic level under water-deficit stress was shown to have an indirect and negative impact on the third trophic level parasitoid, suggesting that parasitoids could be increasingly vulnerable in future warming scenarios.

Vernal freeze damage and genetic variation alter tree growth, chemistry, and insect interactions.

Anticipated consequences of climate change in temperate regions include early spring warmup punctuated by intermittent hard freezes. Warm weather accelerates leaf flush in perennial woody species, potentially exposing vulnerable young tissues to damaging frosts. We employed a 2×6 randomized factorial design to examine how the interplay of vernal (springtime) freeze damage and genetic variation in a hardwood species (Populus tremuloides) influences tree growth, phytochemistry, and interactions with an insect herbivore (Chaitophorus stevensis). Acute effects of freezing included defoliation and mortality. Surviving trees exhibited reduced growth and altered biomass distribution. Reflushed leaves on these trees had lower mass per area, lower lignin concentrations, and higher nitrogen concentrations, altered chemical defence profiles, and supported faster aphid population growth. Many effects varied among plant genotypes and were related with herbivore performance. This study suggests that a single damaging vernal freeze event can alter tree-insect interactions through effects on plant growth and chemistry. Differential responses of various genotypes to freeze damage suggest that more frequent vernal freeze events could also influence natural selection, favouring trees with greater freeze hardiness, and more resistance or tolerance to herbivores following damage.

Sugarcane Aphid Population Growth, Plant Injury, and Natural Enemies on Selected Grain Sorghum Hybrids in Texas and Louisiana.

In response to the 2013 outbreak of sugarcane aphid, Melanaphis sacchari (Zehntner) (Hemiptera: Aphididae), on sorghum, Sorghum bicolor (L.), in North America, experiments were conducted at three southern U.S. grain sorghum production locations (Corpus Christi, TX; Winnsboro, LA; Rosenberg, TX). The objectives were to authenticate yield decline on susceptible hybrids (2014 and 2015) and to measure aphid population growth and natural enemy prevalence on susceptible and resistant hybrids with similar genetic background (2014). Yield decline on susceptible hybrids (Tx 2752/Tx430 and DKS53-67) was more substantial when aphid population growth accelerated quickly and peaked above 300 aphids per leaf (50 to nearly 100% yield decline). Location and year variation in maximum aphid density and cumulative aphid-days was high, with doubling time values on the susceptible hybrids ranging between 3.9 and 7.9 d. On resistant Tx2752/Tx2783, leaf injury and yield decline were not seen or less severe than on its paired susceptible Tx2752/Tx430. Aphids declined on Tx2752/Tx2783 after initial colony establishment (Corpus Christi) or took about 60% longer to double in population size when compared with Tx2572/Tx430 (Winnsboro). The predominant natural enemy taxa were aphelinid mummies (Hymenoptera: Aphelinidae), ladybird beetles (Coleoptera: Coccinellidae), and sryphid flies (Diptera: Syrphidae), and they were more prevalent during flowering than prior to flowering. They were generally responsive to changes in aphid density of both susceptible and resistant hybrids, but variability points to need for further study. In future research, full season observations should continue as well as more detailed study of potential compatibility of sorghum resistance and biological control.

Plant defences limit herbivore population growth by changing predator-prey interactions.

Plant quality and predators are important factors affecting herbivore population growth, but how they interact to regulate herbivore populations is not well understood. We manipulated jasmonate-induced plant resistance, exposure to the natural predator community and herbivore density to test how these factors jointly and independently affect herbivore population growth. On low-resistance plants, the predator community was diverse and abundant, promoting high predator consumption rates. On high-resistance plants, the predator community was less diverse and abundant, resulting in low predator consumption rate. Plant resistance only directly regulated aphid population growth on predator-excluded plants. When predators were present, plant resistance indirectly regulated herbivore population growth by changing the impact of predators on the herbivorous prey. A possible mechanism for the interaction between plant resistance and predation is that methyl salicylate, a herbivore-induced plant volatile attractive to predators, was more strongly induced in low-resistance plants. Increased plant resistance reduced predator attractant lures, preventing predators from locating their prey. Low-resistance plants may regulate herbivore populations via predators by providing reliable information on prey availability and increasing the effectiveness of predators.

Effect of ant attendance on aphid population growth and above ground biomass of the aphid's host plant

Ant-aphid mutualism is considered to be a beneficial association for the individuals concerned. The population and fitness of aphids affected by ant attendance and the outcome of this relationship affects the host plant of the aphid. The main hypothesis of the current study is that ant tending decreases aphid developmental time and/or increases reproduction per capita, which seriously reduces host plant fitness. The effect of attendance by the ant Tapinoma erraticum (Latreille, 1798) (Hymenoptera: Formicidae) on population growth and duration of different developmental stages of Aphis gossypii Glover (Hemiptera: Aphididae) were determined along with the consequences for the fitness of the host plant of the aphid, Vicia faba L., in greenhouse conditions. The initial aphid density was manipulated in order to study aphid performance due to density-dependent changes in ant attendance. The population growth rate of ant attended aphids was more than that of unattended aphids. However, the ratio of 1st-2nd nymphs to adults in aphid populations attended by ants was lower. The yields of bean plants on which the aphids were attended by ants were significantly greater than those of unattended plants. This study indicates that ants not only increase aphid fitness in terms of their population growth rate, but also benefit the host plant.

The Effect of Temperature and Host Plant Resistance on Population Growth of the Soybean Aphid Biotype 1 (Hemiptera: Aphididae).

A laboratory experiment was conducted to evaluate direct and indirect effects of temperature on demographic traits and population growth of biotype 1 of the soybean aphid, Aphis glycines Matsumura. Our objectives were to better understand how temperature influences the expression of host plant resistance, quantify the individual and interactive effects of plant resistance and temperature on soybean aphid population growth, and generate thermal constants for predicting temperature-dependent development on both susceptible and resistant soybeans. To assess indirect (plant-mediated) effects, soybean aphids were reared under a range of temperatures (15-30 °C) on soybean seedlings from a line expressing a Rag1 gene for resistance, and life history traits were quantified and compared to those obtained for soybean aphids on a susceptible soybean line. Direct effects of temperature were obtained by comparing relative differences in the magnitude of life-history traits among temperatures on susceptible soybeans. We predicted that temperature and host plant resistance would have a combined, but asymmetrical, effect on soybean aphid fitness and population growth. Results showed that temperature and plant resistance influenced preimaginal development and survival, progeny produced, and adult longevity. There also appeared to be a complex interaction between temperature and plant resistance for survival and developmental rate. Evidence suggested that the level of plant resistance increased at higher, but not lower, temperature. Soybean aphids required about the same number of degree-days to develop on resistant and susceptible plants. Our results will be useful for making predictions of soybean aphid population growth on resistant plants under different seasonal temperatures.