Occasionally, may weed , Wright’s ground cherry and volunteer tomato survived the 1.5 lb. a.i./acre rate of ethalfluralin. Weed species composition after the 0.75 lb. a.i./acre treatment of ethalfluralin was similar, but also included a few surviving seedlings of lambsquarters. Weed composition on plots that received no herbicide resembled the 1999 weed survey and included barnyard grass, black nightshade, hairy nightshade, redroot pigweed, annual sowthistle, Wright’s ground cherry, cheese weed , purslane and lambs quarters. Generally, higher weed-seedling survival after reduced herbicideapplication rates is typical. Griffin et al. reported lower weed control with reduced rates of soil herbicides in soybean fields. Preplant incorporated application of imazaquin at the full rate gave 95% control, whereas the half rate gave 88% control. Greater seedling survival after reduced herbicide rates may be due to the density thresholds used in this study. For example, Williams et al. used a reduced rate at or below one seedling per square yard of Polygonum aviculare in corn. A relatively high weed-density threshold used for the no-herbicide plots was probably responsible for the low success of the no-herbicide approach in this experiment. The threshold for the zero rate was defined as a seedling density below 10 plants per square yard and for mature plants, as less than one weed plant per square yard. Since treatment maps were based on counts of emerged plants, the threshold for the no-herbicide rate should be set to zero weed plants per square yard. In this experiment, areas treated with the medium rate had about 5% weed cover at 2 and 4 weeks after application and about 12% at 6 weeks; whereas, the high-rate plots had about 2% weed cover at 2 weeks, 5% at 4 weeks and 8% at 6 weeks. Increases in weed cover over time are due to herbicide decomposition in the soil, indoor grow table although ethalfluralin persists for a long time, with an average field half-life of 60 days . High or full herbicide rates should only be applied to high-density weed patches. However, even the full herbicide rate was not able to control weeds in highly infested areas.

Other researchers have also observed that weed clumps persist despite uniform full-rate treatment . High weed-density areas may require a slightly higher rate than what is currently considered full rate, assuming crop tolerance is sufficient. Variablerate herbicide applications could allow higher rates to be applied in high weed density areas, while still applying less herbicide to the field as a whole.When the herbicide application was based on the seedling map, 15% of the experimental area did not receive any herbicide and 63% received a medium rate. The treatment map indicated that 2.18 acres of the site were treated with 0.75 lb. a.i./acre, 0.75 acre with 1.5 lb. a.i./acre, and 0.52 acre with no herbicide. A 47% reduction in herbicide use was achieved with the seedling-map approach when compared with a uniform full-rate application. Reduced rates were applied to 78% of the experimental area. The treatment map that we developed based on mature plants recommended that 1.02 acres of the site be treated with 0.75 lb. a.i./acre, 1.77 acres with 1.5 lb. a.i./acre, and 0.66 acre with no herbicide. Nineteen percent of the experimental area did not receive any herbicide and 30% received the 0.75 lb. a.i./acre rate. A 34% reduction in herbicide use was achieved with the variable-rate application based on a mature-plant weed map when compared with a uniform full-rate application, and 49% of the experimental site received a reduced herbicide treatment. Since using no herbicide may present too much risk for many growers — particularly in the early stages of adoption for precision weed management — rates may be limited to medium and high applications, in which case the herbicide reduction would have been 39% for the seedling-map and 24% for the mature map approach.It took approximately 20 seconds to count mature weeds in each 32-squareyard measurement area. Depending on the level of experience, it would take 2.2 to 6.6 hours to count the weeds in 100 acres . In the variable-rate experiment, a 34% herbicide reduction was achieved with the mature-weed map approach. At a commercial price of $50 per gallon for the herbicide Sonalan, savings were $17. It would take $22 to produce a detailed weed map . In this scenario — based on a mature-weed map — no financial benefits would be achieved.

In the case of the weed-seedling map approach, where a 49% herbicide reduction and $24 herbicide cost savings was achieved, plus the approximate $22 cost of a weed map, variable-rate application brings some modest financial benefits of about $2 per acre. However, we did not account for the conversion of a weed map into an herbicide treatment map in this estimation of economic returns. Our economic analysis should be verified in another study before a firm decision is formed about the economic value of variablerate technology. The economic efficiency of site-specific herbicide application depends on the cost of herbicide, cost of producing the weed map and treatment map, and the spatial characteristics of the weed population. Since weed distribution within a field is slow to change, maps created in one year may be useful for several years. Additionally, there are research efforts currently examining the use of camera systems to mechanically map weeds, which will likely decrease the cost of weed mapping and improve its accuracy, since a greater portion of the field will be sampled.The results from this experiment show that when information about the spatial distribution of the previous year’s mature weeds is used, weed control in terms of subsequent weed cover is comparable to uniform one-rate herbicide application, while simultaneously the total amount of herbicide applied decreases. We conclude that variable-rate spraying based on maps created from estimating weed population density and levels of infestation just before harvest gave the best weed control. However, further improvement is likely when the prediction and modeling of weed-seed redistribution from harvest to application time is incorporated into treatment maps. The simulation of seed movement from the measurement event to herbicide application should be incorporated in any preemergent treatment map.In the modern era of global trade, species are being inadvertently and deliberately introduced widely beyond their historic ranges . A crucial focus of evolution‐ ary ecology of introduced species is to understand their pattern of spread and to identify their native origins and pathways of intro‐ duction to better prevent and manage biological invasions . Inferring the origins and spread of these exotic species is challenging and rarely are the true pathways or origins known. Thus, a fruitful approach may be to use documented intro‐ ductions, such as those performed in classical biological control, as model systems to provide greater insights into population genetic analyses, as well as insight into the consequences of population movement and ecological processes for the genetic structure and variation of a species .

Classical biological control uses natural enemies to control invasive populations of weeds, and arthropod pests and disease vectors in the introduced range . These natural enemies, as biological control agents, are often imported across disjunct geographic ranges for the long‐term control of the target invasive species. In the modern era, these importations are well‐regulated and well documented . Thus, they provide model systems to study the repercussions of invasion pathways and multiple intro‐ ductions—including their effects on inter‐ and intraspecific hybrid‐ ization, bottlenecks, inbreeding, genetic variation, and correlations of genetic diversity with population performance of the biological control agents . To enhance the establishment and success of biological control agents, often multiple separate introductions are made, and large numbers of individuals are released . Multiple introductions here refer to introducing individuals from more than one population, or of more than one species, or both into the same geographic areas. Multiple introductions can increase the genetic diversity in an introduced population due to genetic admix‐ ture of different source populations . Alternatively, multiple introductions of more than one population could interfere with local adaptation, particularly in the native range . Additionally, hybridization can occur when more than one closely related species or strain is introduced, which can poten‐ tially lead to hybrid breakdown or hybrid vigor . Hybrid vigor can result from positive epistatic interac‐ tions among loci, drying rack cannabis heterosis due to masking of deleterious alleles, or heterozygote advantage, whereas hybrid breakdown can occur from negative epistatic effects among loci and/or the under dominance of loci . Thus, the presence of multiple introductions and hybrids can greatly impact the growth and spread of introduced populations, and the efficacy of biological control programs. The introduction of large numbers of individuals is critical to im‐ prove establishment success, as it buffers against demographic sto‐ chasticity and helps minimize loss of genetic variation . Nonetheless, introduced populations often endure demographic bottlenecks , which can decrease allelic richness and heterozygosity, with the latter depending on the rate of population growth following the initial bottleneck . Certain alleles might increase or decrease in frequency by chance during bottlenecks, leading intro‐ duced populations to diverge from native populations . Genetic drift and inbreeding can also lead to increased homozygosity , which can be associated with reduced fitness . However, population bottlenecks do not always reduce genetic variation or lead to genetic differentiation from the native population , particularly if populations grow rapidly following introduction . Evaluating the effects of bottlenecks in population size on genetic diversity can enhance our understanding of the consequences of introductions and spread of species. Although great efforts are taken to introduce many individuals from the native range to enhance establishment success, regulatory processes can make this difficult. Thus, the number of individuals imported to a region ranges widely from 10 to more than 1,000.

While regulations vary by country , each collection from the native range for release typically passes through quarantine to prevent unintentional introductions of other species . In many countries, such as the United States, fur‐ ther screening to characterize host range is often required for each new collection from the native range, which can mean many additional generations in quarantine even for agents that have already been approved. During this time, inbreeding and adaptation to the quarantine and mass‐rearing environment can also occur . Following quarantine screening, population size is typically increased as much as possible in order to release hundreds to thousands of individuals . However, the proportion of individuals that survive in the field and contribute to the next generation may be low, resulting in another demographic bottleneck . Regulatory and logistical obstacles limit sampling from the native range; thus, biological control agents for release in new regionsare often collected from a population already in use for biological control rather than revisiting the native range. This introduction process is analogous to the movement of invasive species, whereby an introduced population becomes the source of several secondary introductions, and is therefore acknowledged as a “bridgehead population” . Similarly, biological control agents frequently undergo serial importation steps, and thus serial bottlenecks in population size. By using the known introduction pathways from biological control programs, we can evaluate our ability to reproduce the introduction pathways by analyzing data from molecular markers. Here, we examine the importation history, genetic diversity, and population structure of two closely related species introduced for biological control to gain insight into the consequences of population movement and ecological processes for the genetic structure and variation of these two species. Here, we ask: Is there evidence of hybridization between these species, and how do introduction processes affect the genetic variation and structure of these species? More specifically, are there indications of decreased heterozygosity and allelic diversity in the introduced populations relative to the native range, do increases in the number of in‐ dividuals initially released or genetic admixture from multiple introductions result in increased genetic diversity, do populations with more introduction steps between them and the source population in the native range exhibit greater loss in genetic variation com‐ pared to populations with fewer introduction steps, and despite originating from the same initial populations, have introduced populations differentiated from the native range and from each other? To address these questions, we use the documented importation history and polymorphic microsatellite loci of two weevils, Neochetina bruchi and N. eichhorniae Hustache from their native and introduced ranges.