Tier one data are the responsibility of the primary research group involved in the project. The key outcome of Tier one is to provide robust measures of the impact of each agent on the target species. To achieve this it is often necessary to identify and quantify key transition- stage relationships for the target (Woodburn and Cullen, 1993; Sheppard et al., 1994; Woodburn and Cullen, 1996; Smyth et al., 1997; Briese, 2000; Sheppard et al,. 2001; Briese et al., 2002). For example, in the case of the annual broad-leafed pasture weed, Echium plantagineum, a suite of four agents have been selected
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(Mogulones larvatus Schultze (Coleoptera: Curculionidae), M. geographicus Goeze (Coleoptera: Curculionidae) and Longitarsus echii Koch (Coleoptera: Chrysomelidae) attack the crown and root zone of the plant during the growing season influencing plant size and therefore seed production. The fourth agent, the pollen beetle, Meligethes planiusculus Heer (Coleoptera: Nitidulidae), attacks the flowering cyme and has a direct effect on seed production.
Collecting seed production data for E. plantagineum is time consuming and labour intensive, so to streamline data collection several key relationships have been developed as a part of Tier one evaluation. The first is the relationship between the numbers of seed produced per plant per unit of flowering cyme length (mm) (Figure 1). Data collection involved counting individual seeds produced in each calyx along the length of a sub sample of flowering cymes per plant, at multiple sites over multiple years. Seed production is also sequential as the cyme elongates throughout the flowering season which requires sampling on a fortnightly basis throughout the season so that no seed are missed (Smyth et al., 1997).
Figure 1. The relationship between E. plantagineum flowering
cyme length and seed per plant (log scale). Data collected from sites at Jugiong and Tarcutta, NSW 1992-1994.
Another relationship was based on the number of seed produced per plant per unit of taproot size, measured as taproot diameter just below the crown (Figure 2). Data collection for this relationship initially involved counting all seeds on a sub sample of cymes to add additional validation to the seed per cyme relationship. Once the first year’s analysis was complete a decision was taken to only measure cyme length in subsequent samples and rely on the seed per cyme length relationship to estimate seed production. Plant weights were also sampled and there is a relationship here between plant weight and taproot diameter as described by the equation below:
Log10 (shoot wt) = 1.643 x Log10(taproot Ø) - 1.0177
Figure 2. The relationship between the tap toot diameter
of E. plantagineum and seed produced per plant. Data collected at 5 sites in NSW, 1992-94.
The data presented provide clear examples of quantifying key plant parameters so that simple plant measures may be used to accurately estimate vegetative or
reproductive output.
Mogulones larvatus At the same time as the plant
relationships were being quantified, research was also being carried out to measure the effects of the crown weevil, M. larvatus, on E. plantagineum size and seed production. This work was initially carried out as a manipulative “garden” experiment and was conducted over two years. The experiment involved comparing seed production of attacked and unattacked plants (a chemical exclusion was used) in the presence or absence of pasture competition (Sheppard et al. 2001), (Figure 3).
Figure 3. Impact of M. larvatus and plant competition on E.
plantagineum seed production
The benefits of conducting a garden experiment are not just restricted to their predictive value for likely agent impact. Controlled impact experiments allow
conformation of plant relationships described above under an increasing level of agent attack. In the case of M. larvatus on E. plantagineum its impact is to reduce plant size and subsequent seeding without changing the plant relationships. Controlling for agent dependent effects on plant parameter relationships is vital if these are to be used sensibly in predicting agent impact. 0 0 0.5 1 1.5 2 2.5 0.5 1 1.5 2 2.5 3 3 3.5 4 3.5 Log10(seeds/plants) y = 0.9434X-0.7848 R2 = 0.827 0 0 10 100 1000 seeds/plants Treatment 10000
Control Competition M. larvatus Comp. + M.1
0 0 0.2 0.4 0.6 0.8 1 0.5 1 1.5 2 2.5 3 1.2 1.4 1.6 3.5 Log10(seeds/plants)
Log10 taproot diameter (mm)
3 3.5
y = 1.9763X-0.4203 R2 = 0.7206
Log10 (cyme length mm)
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The final stage in Tier one evaluation is to draw the link between agent number per unit of host plant and any reduction in plant size, seeding and mortality. Robust relationships here allow simplified sampling of agent impact on its host across the distribution of the weed. In the example of M. larvatus on E. plantagineum, quantifying the percentage of plants attacked in a population provides a reasonable predictor of plant mortality (Figure 4).
Figure 4. Attack rate of M. larvatus on E. plantagineum in
winter and subsequent plant mortality.
Quantifying larval number and plant size greatly improves the accuracy of predicting agent impact (Figure 5). When these additional parameters (larval load and plant size) are measured for field populations, the confidence in predicting plant mortality (or whatever parameter you wish to measure) is significantly improved (Figure 6). This extra detail will also be necessary for agents that have less impact on their host than M. larvatus, i.e. those that do not have the ability to cause plant mortality, but simply limit plant size and subsequent reproductive output. It is worth noting that the estimates of these relationships are not static; as additional data are collected between years and sites, the estimates are continually improved.
Figure 5. Relationship between the number of M. larvatus
larvae and the reduction in E. plantagineum size.
Figure 6. The relationship between M. larvatus larvae number
and E. plantagineum size and subsequent plant mortality. *Plant wt predicted from taproot regression
Mogulones geographicus The root weevil
M. geographicus is an example of an agent whose damage rarely causes plant mortality. Instead, larval damage is more subtle and largely occurs under ground on the taproot, causing a reduction in plant size and reproductive output. The relationship in Figure 7 will allow the prediction of M. geographicus impact.
Figure 7. The relationship between M. geographicus larval
number and the reduction in E. plantagineum size.
Longitarsus echii L. echii is another root feeding agent whose impact in a controlled field experiment has been studied. Like M. larvatus, its impact is to reduce plant size. It does not change the allocation to vegetative and reproductive plant structures (Smyth & Sheppard, 2002). Quantifying the relationship between the number of L. echii larvae and plant size is about to commence in the laboratory and at field sites where larval load is high.
Meligethes planiusculus Tier one evaluation for this
species has not yet commenced as it is the least well established agent in the suite and is having only minor local impact on E. plantagineum. Tier one evaluation will revolve heavily around the relationship described in Figure 1, i.e. changes to the seed/cyme length (or seed per calyx) relationship that will describe its impact. In this instance, the relationship between seed/cyme length will be vital for the quantification of impact, as both adults and larvae of M. planiusculus affect seed 0 Percentage attack Plant mortality 0 20 40 60 80 100 20 40 60 80 100 120 y = 1.3598X - 2 R2 = 0.6839 y = -0.0001X2 - 0.001X + 1.2462 R2 = 0.9543 p<0.001 0 0.5 1 1.5 0 50 100 Number of larvae/plant log10(shoot wt+1) 0 log10(larvae/gm plant wt*) log10(plant morality) 0 0.3 0.6 0.9 1.2 0.5 1 1.5 2 2.5 y = 0.6915Ln(X) +2.0197 R2 = 0.983 p<0.001 y = -0.0001X2 - 0.001X + 1.2568 R2 = 0.9851 p<0.001 0 0.5 1 1.5 0 50 100 Number of larvae/plant log10(shoot wt+1)
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impact by exclusion through either chemical or physical means virtually impossible. If a chemical exclusion is attempted the chemical will only exclude larval feeding damage, while using a cage or similar to exclude M. planiusculus adults will effect pollination (Swirepik et al., 1996).