Tong-Xian Liu

Molecular strategies of plant defense and insect counter-defense

Abstract The prediction of human population growth worldwide indicates there will be a need to substantially increase food production in order to meet the demand on food supply. This can be achieved in part by the effective management of insect pests. Since plants have co‐evolved with herbivorous insects for millions of years, they have developed an array of defense genes to protect themselves against a wide variety of chewing and sucking insects. Using these naturally‐occurring genes via genetic engineering represents an environmentally friendly insect pest‐control measure. Insects, however, have been actively evolving adaptive mechanisms to evade natural plant defenses. Such evolved adaptability undoubtedly has helped insects during the last century to rapidly overcome a great many human‐imposed management practices and agents, including chemical insecticides and genetically engineered plants. Thus, better understanding of the molecular and genetic basis of plant defense and insect counter‐defense mechanisms is imperative, not only from a basic science perspective, but also for biotechnology‐based pest control practice. In this review, we emphasize the recent advance and understanding of molecular strategies of attack‐counterattack and defense‐counter‐defense between plants and their herbivores.

Effects of neem‐based insecticides on beet armyworm (Lepidoptera: Noctuidae)

Abstract Three commercial neem [Azadirachta indica A. Juss (Meliaceae)]‐based insecticides, Agroneem, Ecozin, and Neemix, and a non‐commercial neem leaf powder, were evaluated for oviposition deterrence, antifeedant effect on larvae, and toxicity to eggs and larvae of the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), on cotton leaves in the laboratory. Oviposition deterrence in no‐choice, and two‐ and fivechoice assays, was observed for the neem‐based insecticide treatments when compared with a non‐treated control. Neem‐based insecticides also deterred feeding by beet armyworm larvae. Direct contact with neem‐based insecticides decreased the survival of beet armyworm eggs. Survival of beet armyworm larvae fed for 7 days on leaves treated with neembased insecticides was reduced to 27, 33, 60, and 61% for neem leaf powder, Ecozin, Agroneem, and Neemix, respectively. Possibilities for adoption of neem‐based insecticides in commercial cotton for beet armyworm control are discussed.

Effects of decreased O2 and elevated CO2 on survival, development, and gene expression in cowpea bruchids

Use of modified atmospheres with depleted O(2) and/or elevated CO(2) is an environmentally friendly alternative to currently used fumigants for control of stored grain insect pests. In the present study, we examined the impact of hypoxia and hypercapnia on cowpea bruchids (Callosobruchus maculatus), a storage pest of cowpea and other legumes. Two O(2)/CO(2) combinations were used; (i) 10% O(2)+10% CO(2), (ii) 2% O(2)+18% CO(2). In both cases, N(2) was maintained at 80%, equivalent to normal atmospheric concentration. In ambient atmosphere, the rate of O(2) consumption and CO(2) output at different stages (from low to high) was: eggs≈1st instar<2nd instar≈pupae≈adults<3rd instar<4th instar. When exposed to 10% O(2)+10% CO(2), eggs, larvae and pupae were able to complete development and successfully enter the next developmental stage, although developmental time and mortality varied at different stages. In contrast, more severe hypoxic/hypercapnic treatment, i.e. 2% O(2)+18% CO(2), led to cessation of development of all stages. Effects on eggs and adults were most dramatic as they could only withstand 2-3 days exposure. Further, eggs at early (4-6h old) and later stages (102-104 h old, black-headed) were more susceptible compared to those at intermediate stage (52-54 h old). The 3rd and 4th instar larvae were least sensitive and could survive up to 20 days treatment. To gain some insight into molecular mechanisms underpinning the hypoxic/hypercarpnic response, we performed qPCR reactions on selected metabolic genes involved in TCA cycle and in protein digestion, as well as genes encoding stress-responsive heat shock proteins. Patterns of gene expression and proteolysis suggest that cowpea bruchids suppress their metabolic activity and increase stress tolerance when challenged by O(2) deprivation. Transcript abundance as well as proteolytic activity recovered once normoxic conditions resumed. Taken together, cowpea bruchids were found able to cope with hypoxic and hypercapnic stress. This ability was particularly strong in the late larval stage.

Fitness of Encarsia sophia （Hymenoptera： Aphelinidae） parasitizing Trialeurodes vaporariorum and Bemisia tabaci （Hemiptera： Aleyrodidae）

Abstract  Fitness and efficacy of Encarsia sophia (Girault & Dodd) (Hymenoptera: Aphelinidae) as a biological control agent was compared on two species of whitefly (Hemiptera: Aleyrodidae) hosts, the relatively smaller sweetpotato whitefly, Bemisia tabaci (Gennadius) biotype ‘B’, and the larger greenhouse whitefly, Trialeurodes vaporariorum (Westwood). Significant differences were observed on green bean (Phaseolus vulgaris L.) in the laboratory at 27 ± 2°C, 55%± 5% RH, and a photoperiod of 14: 10 h (L: D). Adult parasitoids emerging from T. vaporariorum were larger than those emerging from B. tabaci, and almost all biological parameters of E. sophia parasitizing the larger host species were superior except for the developmental times of the parasitoids that were similar when parasitizing the two host species. Furthermore, parasitoids emerging from T. vaporariorum parasitized more of these hosts than did parasitoids emerging from B. tabaci. We conclude that E. sophia reared from larger hosts had better fitness than from smaller hosts. Those from either host also preferred the larger host for oviposition but were just as effective on smaller hosts. Therefore, larger hosts tended to produce better parasitoids than smaller hosts.

Antioxidant responses of Propylaea japonica (Coleoptera: Coccinellidae) exposed to high temperature stress.

Temperature is one of the most important environmental factors, and is responsible for a variety of physiological stress responses in organisms. Induced thermal stress is associated with elevated reactive oxygen species (ROS) generation leading to oxidative damage. The ladybeetle, Propylaea japonica (Thunberg) (Coleoptera: Coccinellidae), is considered a successful natural enemy because of its tolerance to high temperatures in arid and semi-arid areas in China. In this study, we investigated the effect of high temperatures (35, 37, 39, 41 and 43 °C) on the survival and activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), peroxidases (POD), glutathione-S-transferases (GST), and total antioxidant capacity (TAC) as well as malondialdehyde (MDA) concentrations in P. japonica adults. The results indicated that P. japonica adults could not survive at 43 °C. CAT, GST and TAC were significantly increased when compared to the control (25 °C), and this played an important role in the process of antioxidant response to thermal stress. SOD and POD activity, as well as MDA, did not differ significantly at 35 and 37 °C compared to the control; however, there were increased levels of SOD, POD and MDA when the temperature was above 37 °C. These results suggest that thermal stress leads to oxidative stress and antioxidant enzymes play important roles in reducing oxidative damage in P. japonica adults. This study represents the first comprehensive report on the antioxidant defense system in predaceous coccinellids (the third trophic level). The findings provide useful information for predicting population dynamics and understanding the potential for P. japonica as a natural enemy to control pest insects under varied environmental conditions.

Impact of insecticides on parasitoids of the leafminer, Liriomyza trifolii, in pepper in south Texas

Abstract Liriomyza leafminers (Diptera: Agromyzidae) are cosmopolitan, polyphagous pests of horticultural plants and many are resistant to insecticides. Producers in South Texas rely on insecticides as the primary management tool for leafminers, and several compounds are available. The objective of this study is to address the efficacy of these compounds for controlling Liriomyza while minimizing their effects against natural enemies. Research plots were established at Texas AgriLife research center at Weslaco, Texas in fall 2007 and spring 2008 seasons, and peppers were used as a model crop. Plots were sprayed with novaluron, abamectin, spinetoram, lambda-cyhalothrin and water as treatments according to leafminer infestation; insecticide efficacy was monitored by collecting leaves and infested foliage. Plant phenology was also monitored. Novaluron was the most effective insecticide and lambda-cyhalothrin showed resurgence in leafminer density in fall 2007 and no reduction in spring 2008. Other compounds varied in efficacy. Novaluron showed the least number of parasitoids per leafminer larva and the lowest parasitoid diversity index among treatments followed by spinetoram. Liriomyza trifolii (Burgess) was the sole leafminer species on peppers, and 19 parasitoid species were found associated with this leafminer. Application of these insecticides for management of leafminers with conservation of natural enemies is discussed.

Mating and host density affect host feeding and parasitism in two species of whitefly parasitoids

Abstract  The parasitoids in the genera of Encarsia and Eretmocerus (Hymenoptera: Aphelinidae) are important biological control agents of whiteflies, and some of them not only parasitize hosts but also kill them with strong host‐feeding capacity. Two whitefly parasitoid species, Encarsia sophia and Eretmocerus melanoscutus were examined to determine if mating and host density affected their host feeding and parasitism. The whitefly host, Bemisia tabaci, was presented to these two wasp species in densities of 10, 20, 30, 40, 50 and 60 third‐instar nymphs per clip cage. Mated whitefly parasitoid females fed on more hosts than unmated females under a range of host densities (under all six host densities for En. sophia; under the densities of 40 nymphs or more for Er. melanoscutus). Meanwhile, mated females parasitized more whitefly nymphs than unmated females under all host densities for both species. With increase of host density, mated or unmated Er. melanoscutus females killed more hosts by host feeding and parasitism. Mated En. sophia females killed more hosts by host feeding with increase of host density, whereas unmated females did not parasitze whitefly nymphs at all. Our results suggest that only mated female parasitoids with host‐feeding behavior should be released in crop systems to increase their bio‐control efficiency.

Jasmonate‐ and salicylate‐induced defenses in wheat affect host preference and probing behavior but not performance of the grain aphid, Sitobion avenae

 Jasmonate‐ and salicylate‐mediated signaling pathways play significant roles in induced plant defenses, but there is no sufficient evidence for their roles in monocots against aphids. We exogenously applied methyl jasmonate (MeJA) and salicylic acid (SA) on wheat seedlings and examined biochemical responses in wheat and effects on the grain aphid, Sitobion avenae (Fab.). Application of MeJA significantly increased levels of wheats polyphenol oxidase, peroxidase and proteinase inhibitor 1, 2 and 6 days after treatment. In two‐choice tests, adult aphids preferred control wheat leaves to MeJA‐ or SA‐treated leaves. Electrical penetration graph (EPG) recordings of aphid probing behavior revealed that on MeJA‐treated plants, the duration of aphids first probe was significantly shorter and number of probes was significantly higher than those on control plants. Also total duration of probing on MeJA‐treated plants was significantly shorter than on control plants. Total duration of salivation period on SA‐treated plants was significantly longer, while mean phloem ingestion period was significantly shorter than on control plants. However, no significant difference in total duration of phloem sap ingestion period was observed among treatments. The EPG data suggest that MeJA‐dependent resistance factors might be due to feeding deterrents in mesophyll, whereas the SA‐mediated resistance may be phloem‐based. We did not observe any significant difference of MeJA and SA application on aphid development, daily fecundity, intrinsic growth rate and population growth. The results indicate that both MeJA‐ and SA‐induced defenses in wheat deterred S. avenae colonization processes and feeding behavior, but had no significant effects on its performance.

Aphid-induced plant volatiles affect the attractiveness of tomato plants to Bemisia tabaci and associated natural enemies

An in‐depth understanding of plant‐mediated interactions between herbivores and their natural enemies is essential in community ecology and co‐evolution, and for developing sustainable pest management strategies. The influence of Myzus persicae (Sulzer) (Hemiptera: Aphididae)‐induced tomato plant [Solanum lycopersicum L. (Solanaceae)] volatile compounds on the olfactory responses of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), three predator species [Coccinella septempunctata L., Propylea japonica (Mulstant) (both Coleoptera: Coccinellidae), and Orius sauteri (Poppius) (Hemiptera: Anthocoridae)], two whitefly parasitoid species [Encarsia formosa (Gahan) and Encarsia sophia (Girault & Dodd) (Hymenoptera: Aphelinidae)], and one aphid parasitoid species [Aphidius gifuensis Ashmead (Hymenoptera: Aphidiidae)] were examined in two‐choice tests using a four‐arm olfactometer. Tomato plants were experimentally manipulated for aphid density, duration of aphid feeding, and interval between aphid removal and the behavioral test. We also compared the systemic effects of aphid feeding on the olfactory behavior of whiteflies and natural enemies according to three relative leaf positions: the aphid‐infested leaf itself and two adjacent leaves (above and below). Bemisia tabaci were deterred by the odors of the leaves infested with aphids in all treatments. All six natural enemies were attracted to the leaves infested with aphids. Bemisia tabaci deterrence by volatiles was greater for leaves at the highest aphid density, and with increasing duration of exposure to aphid feeding. In contrast, the attraction to leaves was greater for the three parasitoids than for the three predators both with increasing aphid density and increasing duration of plant exposure to aphids. There was no significant influence of leaf position on preference of B. tabaci, the two species of lady beetles, and the aphid parasitoid. The two Encarsia species were attracted to the leaf above the aphid‐infested leaves, indicating that systemic volatiles were released by adjacent leaves. On the other hand, O. sauteri was only attracted by the aphid‐infested leaf itself. Our results showed that M. persicae‐infested host plants emitted volatiles that could inhibit the colonization by B. tabaci and also attract natural enemies of both aphids and whiteflies. The results of this study may need to be considered further for optimizing pest management methods.

Life history and life table analysis of the whitefly predator Delphastus catalinae (Coleoptera: Coccinellidae) on collards

Abstract The ladybeetle, Delphastus catalinae (Horn), is one of the most commonly used predacious natural enemies being commercially reared for controlling whiteflies, including Bemisia tabaci (Gennadius) biotype B (=B. argentifolii Bellows & Perring), on various ornamental and vegetable crops under greenhouse conditions. The development, survivorship, and fecundity of D. catalinae feeding on B. tabaci biotype B on collard plants were determined in the laboratory, and the age‐specific life table parameters were analyzed based on the life history data. Developmental time was 4.0, 1.9, 1.1, 1.4, 5.2, and 5.3 days for eggs, first, second, third, fourth instars, and pupae, respectively, with an average of 18.9 days from oviposition to adult emergence for both sexes, 19.0 days for females, and 18.8 days for males. Adult longevities averaged 146.6 days for both sexes, 122.6 days for females, and 170.5 days for males. After an average 4.9 days preoviposition period, females laid a mean of 5.6 eggs per day over a 97.0‐day period. Net reproductive rate (R0) and gross reproductive rate (ΣMmx) were estimated by life table analysis at 276.8 and 325.1, respectively. Generation time (T) and doubling time (DT) were 35.6 and 4.8 days respectively, and the intrinsic rate of natural population increase (rm) was estimated at 0.158, or l= 1.171 for the finite rate of increase. The rm value of D. catalinae is similar to or higher than those of the whitefly feeding on most vegetable and ornamental crops, indicating that the ladybeetle is capable of regulating populations of B. tabaci biotype B and other whiteflies under greenhouse conditions.