Thomas W. Sappington

Western corn rootworm (Diabrotica virgifera virgifera LeConte) population dynamics

1u2002The western corn rootworm Diabrotica virgifera virgifera LeConte is a major insect pest of field maize, Zea mays L. Larvae can cause substantial injury by feeding on maize roots. Larval feeding may destroy individual roots or root nodes, and reduce plant growth, stability, and yield. Costs associated with managing corn rootworms in continuous maize are annually one of the largest expenditures for insect management in the United States Corn Belt.

Distribution of Genes and Repetitive Elements in the Diabrotica virgifera virgifera Genome Estimated Using BAC Sequencing

Feeding damage caused by the western corn rootworm, Diabrotica virgifera virgifera, is destructive to corn plants in North America and Europe where control remains challenging due to evolution of resistance to chemical and transgenic toxins. A BAC library, DvvBAC1, containing 109,486 clones with 104 ± 34.5u2009kb inserts was created, which has an ~4.56X genome coverage based upon a 2.58u2009Gb (2.80u2009pg) flow cytometry-estimated haploid genome size. Paired end sequencing of 1037 BAC inserts produced 1.17u2009Mb of data (~0.05% genome coverage) and indicated ~9.4 and 16.0% of reads encode, respectively, endogenous genes and transposable elements (TEs). Sequencing genes within BAC full inserts demonstrated that TE densities are high within intergenic and intron regions and contribute to the increased gene size. Comparison of homologous genome regions cloned within different BAC clones indicated that TE movement may cause haplotype variation within the inbred strain. The data presented here indicate that the D. virgifera virgifera genome is large in size and contains a high proportion of repetitive sequence. These BAC sequencing methods that are applicable for characterization of genomes prior to sequencing may likely be valuable resources for genome annotation as well as scaffolding.

Genes, gene flow and adaptation of Diabrotica virgifera virgifera

1u2002Diabrotica virgifera virgifera has emerged as a major pest of cultivated maize, due to a combination of its high capacity to inflict economic damage, adaptability to pest management techniques and invasiveness.

Beet Armyworm (Lepidoptera: Noctuidae) Host Plant Preferences for Oviposition

Abstract Beet armyworm, Spodoptera exigua (Hübner), oviposition preferences were determined on five host plants: cabbage (Brassica oleracea capitata L.), cotton (Gossypium hirsutum L.), bell pepper (Capsicum annuum L.), pigweed (Amaranthus retroflexus L.), and sunflower (Helianthus annuus L.) in no-choice, two-choice, and five-choice tests. Tests were conducted in the laboratory, greenhouse, and field cages. Oviposition preferences were compared on the basis of two measurements, the proportion of eggs laid on the plants to total that were deposited, and the oviposition preference index defined as [(number of eggs laid on the plant) - (number of eggs laid on the cage)] × 100/total number of eggs laid. The proportion of eggs laid on the plants to total that were deposited was highest for pigweed and lowest for cabbage in all tests. Beet armyworm females were significantly deterred from laying eggs on cabbage and sunflower, while pigweed and cotton elicited a positive oviposition preference. Pepper tended to be neutral or slightly unattractive. Apparent interactions among plant species in choice tests produced measurable shifts in oviposition preference. Most notably, female response to pepper was enhanced in the presence of cotton or pigweed. Egg masses laid on the plants contained significantly higher numbers of eggs than those laid on the surface of the cage, except in the case of cabbage leaves. Knowledge of hierarchies of host plant oviposition preference by beet armyworm females will be useful in understanding the population dynamics of this important agricultural pest, and for developing effective monitoring and management strategies.

The major yolk proteins of higher Diptera are homologs of a class of minor yolk proteins in lepidoptera.

In most oviparous animals, including insects, vitellogenin (Vg) is the major yolk protein precursor. However, in the higher Diptera (cyclorrhaphan flies), a class of proteins homologous to lipoprotein lipases called yolk polypeptides (YP) are accumulated by oocytes instead of Vg, which is not produced at all. Lepidopterans (moths) produce Vg as the major yolk protein precursor, but also manufacture a class of minor yolk proteins referred to as egg-specific proteins (ESP) or YP2s. Although the lepidopteran ESP/YP2s are related to lipoprotein lipases, previous attempts to directly demonstrate their homology with higher-dipteran YPs were unsuccessful. In this paper, a multiple alignment of amino acid sequences was constructed using a shared lipid binding motif as an anchor, to demonstrate that lepidopteran ESP/YP2s, higher-dipteran YPs, and lipoprotein lipases are indeed homologous. Phylogenetic analyses of the aligned sequences were performed using both distance-based and parsimony strategies. It is apparent that the higher dipterans did not requisition a lipoprotein lipase to replace Vg as a yolk protein precursor, but instead utilize a class of proteins with an evolutionary history of use as minor constituents of yolk in other insects.n

Synchronized Oviposition Triggered by Migratory Flight Intensifies Larval Outbreaks of Beet Webworm

Identifying the reproductive consequences of insect migration is critical to understanding its ecological and evolutionary significance. However, many empirical studies are seemingly contradictory, making recognition of unifying themes elusive and controversial. The beet webworm, Loxostege sticticalis L. is a long-range migratory pest of many crops in the northern temperate zone from 36°N to 55°N, with larval populations often exploding in regions receiving immigrants. In laboratory experiments, we examined (i) the reproductive costs of migratory flight by tethered flight, and (ii) the reproductive traits contributing to larval outbreaks of immigrant populations. Our results suggest that the beet webworm does not initiate migratory flight until the 2nd or 3rd night after emergence. Preoviposition period, lifetime fecundity, mating capacity, and egg hatch rate for adults that experienced prolonged flight after the 2nd night did not differ significantly from unflown moths, suggesting these traits are irrelevant to the severity of beet webworm outbreaks after migration. However, the period of first oviposition, a novel parameter developed in this paper measuring synchrony of first egg-laying by cohorts of post-migratory females, for moths flown on d 3 and 5 of adulthood was shorter than that of unflown moths, indicating a tightened time-window for onset of oviposition after migration. The resulting synchrony of egg-laying will serve to increase egg and subsequent larval densities. A dense population offers potential selective advantages to the individual larvae comprising it, whereas the effect from the human standpoint is intensification of damage by an outbreak population. The strategy of synchronized oviposition may be common in other migratory insect pests, such as locust and armyworm species, and warrants further study.

Increased Long-Flight Activity Triggered in Beet Armyworm by Larval Feeding on Diet Containing Cry1Ac Protoxin

Evaluating ecological safety and conducting pest risk analysis for transgenic crops are vitally important before their commercial planting. The beet armyworm, Spodoptera exigua, a long-distance migratory insect pest, is not a direct target of transgenic Cry1Ac-expressing cotton in China, but nevertheless it has recently become an important pest. Migrants leaving their natal field arrive in other appropriate habitat far away in a short time, often followed by larval outbreaks. S. exigua has low susceptibility to Cry1Ac. However, our results from laboratory experiments identified (i) sublethal effects of Cry1Ac protoxin on larval development rate, larval and pupal weight, and adult lifetime fecundity, and (ii) increased long-flight behavior triggered by Cry1Ac which may contribute to larval outbreaks elsewhere. No significant differences in larval mortality, pupation rate, adult emergence rate, longevity, pre-oviposition period, or oviposition period were observed between controls and larvae fed on artificial diet incorporating a low concentration of Cry1Ac protoxin. The negative sublethal effects on some developmental and reproductive traits and lack of effect on others suggest they do not contribute to the observed severity of S. exigua outbreaks after feeding on Cry1Ac cotton. Interestingly, the percentage of long fliers increased significantly when larvae were reared on diet containing either of two low-dose treatments of Cry1Ac, suggesting a possible increased propensity to disperse long distances triggered by Cry1Ac. We hypothesize that negative effects on development and reproduction caused by Cry1Ac in the diet are offset by increased flight propensity triggered by the poor food conditions, thereby improving the chances of escaping adverse local conditions before oviposition. Increased long-flight propensity in turn may amplify the area damaged by outbreak populations. This phenomenon might be common in other migratory insect pests receiving sublethal doses of Bt toxins and warrants further study.

Effects of burial and soil condition on postharvest mortality of boll Weevils (Coleoptera: Curculionidae) in fallen cotton fruit.

Abstract Effects of soil condition and burial on boll weevil, Anthonomus grandis grandis Boheman, mortality in fallen cotton, Gossypium hirsutum L., fruit were assessed in this study. During hot weather immediately after summer harvest operations in the Lower Rio Grande Valley of Texas, burial of infested fruit in conventionally tilled field plots permitted significantly greater survival of weevils than in no-tillage plots. Burial of infested squares protected developing weevils from heat and desiccation that cause high mortality on the soil surface during and after harvest in midsummer and late summer. A laboratory assay showed that burial of infested squares resulted in significantly greater weevil mortality in wet than in dry sandy or clay soils. Significantly fewer weevils rose to the soil surface after burial of infested bolls during winter compared with bolls set on the soil surface, a likely result of wetting by winter rainfall. A combination of leaving infested fruit exposed to heat before the onset of cooler winter temperatures and burial by tillage when temperatures begin to cool might be an important tactic for reducing populations of boll weevils that overwinter in cotton fields.

Proliferation and copy number variation of BEL-like long terminal repeat retrotransposons within the Diabrotica virgifera virgifera genome.

The proliferation of retrotransposons within a genome can contribute to increased size and affect the function of eukaryotic genes. BEL/Pao-like long-terminal repeat (LTR) retrotransposons were annotated from the highly adaptable insect species Diabrotica virgifera virgifera, the western corn rootworm, using survey sequences from bacterial artificial chromosome (BAC) inserts and contigs derived from a low coverage next-generation genome sequence assembly. Eleven unique D. v. virgifera BEL elements were identified that contained full-length gag-pol coding sequences, whereas 88 different partial coding regions were characterized from partially assembled elements. Estimated genome copy number for full and partial BEL-like elements ranged from ~8 to 1,582 among individual contigs using a normalized depth of coverage (DOC) among Illumina HiSeq reads (total genome copy number ~8,821). BEL element copy number was correlated among different D. v. virgifera populations (R2=0.9846), but individual element numbers varied≤1.68-fold and the total number varied by ~527 copies. These data indicate that BEL element proliferation likely contributed to a large genome size, and suggest that differences in copy number are a source of genetic variability among D. v. virgifera.

Reproductive Potential of Overwintering, F1, and F2 Female Boll Weevils (Coleoptera: Curculionidae) in the Lower Rio Grande Valley of Texas

Abstract The feeding and oviposition activity of overwintering boll weevils, Anthonomus grandis grandis (Boheman), and seasonal fluctuations in development, survival, and reproduction of progeny of overwintering and first- and second-generation boll weevil females were determined in the laboratory at 27°C, 65% RH, and a photoperiod of 12:12 (L:D) h. During the cotton-free period in the Lower Rio Grande Valley, female boll weevils without access to cotton resorb their unlaid eggs and enter reproductive diapause. However, when they were provided daily with greenhouse-grown cotton squares, commencement of oviposition began after 7, 15, or 20 d, depending on when they were captured. Females captured later in the winter fed longer before laying eggs than those captured in the early fall, suggesting that it may take females longer to terminate diapause the longer they have been dormant. The rate of feeding by females was significantly less during the winter months, and this may have affected the rate of diet-mediated termination of dormancy. Females of the first and second generations after the overwintering generation produced a significantly higher percentage of progeny surviving to adulthood and a higher proportion of these progeny were females. Offspring development time from overwintering female parents was significantly longer than that from first and second generations under the same laboratory conditions. The total number of lifetime eggs produced by females of the second generation during the cotton-growing season were ≈9.9-fold higher than for overwintering females and 1.5-fold higher than for first-generation females. Life table calculations indicated that the population of second-generation boll weevils increased an average of 1.5-fold higher each generation than for females of the first generation and 22.6-fold higher than for overwintering females. Our data showed variation in boll weevil survival, development, and reproductive potential among the overwintering and first- and second-generation females, suggesting inherent seasonal fluctuations in these parameters.