Comparing actual and perceived soybean defoliation with field surveys and grower estimates

In many flowering plants, flowers contain more ovules than fruits have seeds. What determines which ovules become seeds? When photosynthates are limited, as may happen when plants lose leaf area to herbivory, fewer fertilized ovules become seeds. Greenhouse-grown ramets of distinct individuals of a perennial herbaceous legume were manually defoliated to various levels determined in the field, then self- or cross-pollinated. For each seed produced, we recorded its position in the fruit and its mass. From a subset of seeds from different treatments and positions in the fruits, we grew seedlings and measured their dry mass. Ovules were aborted more frequently in fruits from flowers that were self-pollinated and from those on plants with higher levels of defoliation. Ovules in the basal portion of the fruits were more likely to be aborted than those at the stigmatic end; this pattern was most pronounced for fruits after self-pollination with high levels of defoliation. Total number of seeds produced and seed mass per pod were greatest in cross-pollinated fruits after no or low levels of defoliation. Mean individual seed mass was greater for fruits with fewer seeds, indicating a trade-off between seed number and seed mass. Seedling dry mass (a measure of vigor) was greatest for seeds in the middle positions of fruit produced by cross-pollination after severe herbivory; no positional differences were seen for seeds from self-pollinated fruits. Observed locations of seed abortion may have been selected not only by defoliation, but in part by propensity for dispersal, while positional differences in seedling vigor may be related to seed size and differential maternal allocation based on pollination treatment and leaf area lost.

The relationship between Irish potato tuber yield, Solanum tuberosum L., and defoliation by the Colorado potato beetle, Leptinotarsa decemlineata (Say), was investigated and the use of regression and mean separation analyses as a basis for developing economic injury levels were compared. The type of statistical analysis used to describe this relationship greatly affected the interpretation of data. Mean separation (=multiple-comparison) procedures were inadequate to detect accurately defoliation levels tolerated by potato, because these procedures did not have significant statistical power to detect small reductions in yield. Similarly, regression analyses failed to identify a level of defoliation tolerable to potato because a consistent biological relationship between yield and defoliation could not be determined. For example, a quadratic term (nonlinear) in the analysis of variance was significant in only 2 of 8 experiments, and the relationship between 'Atlantic' and 'Superior' yield and defoliation in these experiments was concave, indicating that yield losses were greatest at low levels of defoliation. A plateau model (nonlinear) suggested that Atlantic potato could tolerate low levels of defoliation (1.8-13% in 4 of 6 data sets), but it did not fit data for the variety Superior. Although a negative linear model provided a statistically significant fit to 7 of 8 data sets (average slopes [±SEM] for Atlantic and Superior were -0.12 ± 0.01 and -0.15 ± 0.03 T/ha per the percentage of defoliation, respectively), it is likely that a linear model underestimates the actual level of defoliation that potato can tolerate, especially at low levels. The implications of these findings for the development of economic injury levels and economic thresholds for the Colorado potato beetle on potato are discussed.

Modelling the impact of defoliation by the leaf beetle, Chrysophtharta bimaculata (Coleoptera: Chrysomelidae), on height growth of Eucalyptus regnans

Eight potato (Solanum tuberosum L.) lines and a control (cv. Russet Burbank) were screened for suitability to Colorado potato beetle, Leptinotarsa decemlineata (Say) (Chrysomelidae), and for defoliation levels in the field to determine whether laboratory‐obtained measures of performance are useful for predicting defoliation. Several lines caused reduced larval growth rates and survival compared to the control. Forty‐eight hour weight gains by fourth instar larvae, after adjustment for consumption rates, were reduced on three lines, suggesting that these lines had antibiosis‐like effects. Lines having very low suitability in the laboratory showed very low defoliation levels in the field, whereas lines that were highly suitable in the laboratory tended to suffer extensive defoliation in the field. Correlation analyses suggested that fourth instar growth rates or larval survival rates are useful measures for predicting defoliation in the field.

Variation in in situ growth performance of the mountain birch as indicated by the widths of annual rings was analysed and related mainly to temperature and herbivory using ring width series from five heath forest sites in the Lake Torneträsk area, northern Sweden. Climate explained 48–64% of the variation in age‐corrected mean ring width series. In general, the effect of current year July followed by June temperature was most important at all sites. A warm May resulted in wider rings due to an earlier budburst. Short‐term (inter‐annual) responses to increased temperature were in most cases not reflected into long‐term responses (decades). A large proportion of the variation in stem mean ring width was due to variation among stems within trees (81%) in these polycormic trees, while variation among sites was marginal (0.4%). Within trees, main stems grew faster and were more responsive to climate variation than subordinate stems. No effect of insect herbivory on ring width was found at low defoliation levels (≤12%). At a defoliation level of ca 84% a one‐year reduction in stem growth was observed while the growth reduction (ca 50% reduction in ring width) lasted for 4 yr after ca 93% defoliation. After outbreaks resulting in complete defoliation and some stem mortality, ring widths of surviving stems mainly responded with increased growth. Basal sprouts, emerging just after a severe insect outbreak with a high mortality of old stems, grew faster than sprouts occurring during other periods. It is concluded that the mountain birch is well adapted to recover from Epirrita outbreaks; the ability to produce basal sprouts, that can benefit from an existing root system for fast initial growth, is one important mechanism for this.

Analysis of high resolution multispectral MEIS imagery for spruce budworm damage assessment on a single tree basis

Gonipterus scutellatus outbreaks may severely defoliate Eucalyptus plantations growing in South Africa. Therefore, detecting and mapping the severity and extent of G. scutellatus defoliation is essential for the deployment of suppressive measures. In this study, we tested the utility of spatially optimized vegetation indices and an artificial neural network in detecting and mapping G. scutellatus-induced vegetation defoliation, using both visual estimates of percentage defoliation and optical leaf area index (LAI) measures. We tested both field methods to determine which of the two were more superior in detecting vegetation defoliation using optimized vegetation indices. These indices were computed from a WorldView-2 pan-sharpened image, which is characterized with a 0.5-m spatial resolution and eight spectral bands. The indices were resampled to spatial resolutions that best represented levels of G. scutellatus-induced defoliation. The results showed that levels of defoliation, using visual percentage estimates, were detected with an R2 of 0.83 and an RMSE of 1.55 (2.97% of the mean measured defoliation), based on an independent test data-set. Similarly, LAI subjected to defoliation was detected with an R2 of 0.80 and an RMSE of 0.03 (0.06% of the mean measured LAI), based on an independent test data-set. Therefore, the results indicate that the cheaper less-complicated visual percentage estimates of defoliation was the more superior model of the two. A sensitivity analysis revealed that NDRE, MCARI2 and ARI ranked as the top three most influential indices in developing both percentage defoliation and LAI models. Furthermore, we compared the optimized model with a model developed using the original image spatial resolution. The results indicated that the optimized model performed better than the original 0.5-m spatial resolution model. Overall, the study showed that vegetation indices optimized to specific spatial resolutions can effectively detect and map levels of G. scutellatus-induced defoliation and LAI subjected to defoliation.

Research has been conducted on the response of determinate and indeterminate soybean [Glycine max(L.) Merr.] to various plant‐damage treatments, but information is limited on the response of genotypes with a semideterminate growth habit. Our objective was to determine the yield response of a determinate cv., Forrest, and a semideterminate strain, R74‐334, to different levels of defoliation and plant cutoff at two growth stages. Treatments consisted of plants being cut off at half height, cut off at half height and defoliated below the cut, or completely defoliated. The treatments were applied to 25, 50, 75, or 100% of the plants at growth stages V5 and R3. These field experiments were conducted on a Captina silt loam (fine‐silty, mixed, mesic Typic Fragiudult) during two growing seasons. Mean yields of Forrest were significantly higher than those of R74‐334 during both years. Most of the treatments at V5 did not significantly decrease yield and in three treatments significantly increased yield. Yields generally were significantly reduced at R3 by treatments involving 50% or greater plant damage. The greatest reduction in yield occurred when 100% of the plants were cut off at half height and then completely defoliated. Yield of Forrest was reduced by 51% and that of R74‐334 by 44% with this treatment. The yield of both genotypes was reduced 33% by 100% defoliaton at stage R3. None of the first order interactions involving genotypes was statistically significant, which indicates that the two genotypes responded similarly to these plant‐damage treatments. Although there was some variability in the response of the genotypes to certain treatments, the data indicate that determinate and semideterminate soybean recover about the same from plant cutoff and/or defoliation. Therefore, the same loss values that have been developed for determinate soybean probably should be used for semideterminate types.

Soybean response to weed interference and defoliation

Following a 3-year period of high populations of the little spruce sawfly,Pristiphora abietinaChrist, a monophagous species infesting Norway spruce,Picea abiesKarst., we compared variations in foliar nutrients (nitrogen, water, soluble carbohydrates, starch) and potential defensive compounds (fiber, quinic and shikimic acids, tannins) of trees that had been subjected to high or very low levels of defoliation. During the time of needle expansion a substantial decline in leaf quality occurred: nitrogen, water, and carbohydrate levels decreased, whereas fiber and starch concentrations increased. Allelochemicals such as tannins and organic acids, however, peaked at bud break and then rapidly declined. More heavily attacked trees had higher nitrogen, water, and fiber contents but lower starch and organic acid concentrations than lightly attacked trees. The needle tannin content was significantly lower in heavily attacked trees throughout the period of larval feeding. Needle tannin levels and the nitrogen to tannin ratios seem to be the most probable traits to distinguish sawfly-resistant from sawfly-susceptible Norway spruce trees.

Gypsy moth defoliation of oak trees has been shown to lead to increased tannin levels, which, in turn, lead to reduced gypsy moth growth and fecundity. In laboratory experiments, increased tannin levels can interfere with the transmission of a virus that is consumed by larvae on oak foliage, and high mortality rates of larvae in the field are sometimes associated with low levels of defoliation. These latter results have led to the suggestion that gypsy moth defoliation may cause reduced mortality attributable to the virus by elevating oak tannin levels. In a series of field experiments, we directly tested the hypothesis that gypsy moth defoliation of oaks leads to reduced virus transmission rates. In each of three study years, in oak forests with almost no naturally occurring gypsy moths or virus, we measured virus transmission rates in gypsy moths feeding on oaks, with and without experimental defoliation. By carefully synchronizing our experiments with the phenology of natural gypsy moth populations, we mimicked natural virus transmission processes during that part of the gypsy moth life cycle when virus transmission occurs. In our experiments, there was no effect of gypsy moth defoliation on tannin levels; consequently, virus transmission in both the field and the lab was unaffected by defoliation. Although we did observe increased tannin levels on more severely defoliated oak trees in one of two naturally defoliated oak stands late in the season, virus transmission had virtually ceased by that time. Our results suggest that gypsy moth defoliation does not affect tannin levels early enough in the larval season to have a measurable effect on the interaction between the gypsy moth and its nuclear polyhedrosis virus.

Tangolagrass (Brachiaria arrecta × Brachiaria mutica) is a stoloniferous warm-season grass considered as an alternative to forming permanent grasslands in waterlogged tropical regions. However, information about grazing management targets for such species is still scarce. This two-year study aimed to identify pre-grazing canopy heights that do not compromise both leaf lamina production and nutritive value of tangolagrass pastures, and to test whether lower canopy height could decrease stolon elongation process. To this end, three pre-grazing canopy heights (20, 30, or 40 cm, all lowered in 40% of their initial heights) were assigned to nine 390-m2 plots grazed by cattle. Herbage, leaf, and stem accumulation rates were not different among treatments (57, 23, and 27 kg DM ha−1 d−1, respectively; p > 0.05). Also, neutral detergent fiber (NDF) and acid detergent fiber (ADF) were similar among treatments (56 and 24% DM, respectively; p > 0.05), but crude protein (CP) was greater (p = 0.0180) in pastures managed with 30 cm (24.7% DM × 22.5% DM). Thus, pre-grazing canopy heights between 20–40 cm combined with low levels of defoliation (up to 40% of the initial height) provide the same primary productivity in tangolagrass pastures. However, stolon elongation could not be reduced within such canopy height targets.

A natural population of the armyworm Mythimna separata , attacking maize at the time of silking caused a significant reduction in yield only on plants suffering greater than 67% defoliation. At this level of leaf loss, yield was reduced by 44%, mainly because of a reduction in seed weight. Direct damage to cobs was negligible. The upper leaves were only seriously defoliated after the lower part of the plant had been largely consumed. There was a small compensatory increase in weight of the upper leaves, seed number, and yield at low levels of defoliation (11–33%). This is attributed to the observed preference of M. separata larvae for the lower parts of the plant, and the results of other research showing that the upper leaves can compensate most effectively for plant damage. An analysis of the cost of chemical control suggests that it is rarely economical.

Summary1. Interactions between primary producers and consumers (i.e. grazers) are of fundamental importance to the successful functioning of ecological communities. Plant–herbivore interactions have been extensively studied, and herbivory has been accepted as an important process contributing to the structure of terrestrial and aquatic ecosystems. In contrast, the functional importance of the ecologically equivalent interaction between scleractinian reef corals and polyp‐feeding fishes is largely untested, but has generally been dismissed as unimportant.2. This study quantified the amount of tabular acroporid coral tissue biomass consumed at the population level by corallivorous butterflyfishes and determined the proportion of both the standing biomass and productivity that is consumed annually at three exposed reef crest sites at Lizard Island, Great Barrier Reef and Australia.3. Total daily coral consumption ranged from 18·6 (±1·6) to 27·4 (±1·5) g 200 m−2 day−1 with 61–68% of this consumption directed towards tabular acroporid corals. This selective feeding resulted in an annual consumption of between 8·9–13·5% of the total available tissue biomass and between 52–79% of the annual productivity of these tabular acroporid corals.4. The proportion of standing coral tissue biomass removed by corallivorous butterflyfishes is similar to that removed from terrestrial plants by herbivores. However, the proportion of primary productivity consumed is considerably greater on coral reefs for both corallivorous and herbivorous fishes compared with terrestrial systems.5. In terrestrial systems, even relatively low levels of defoliation can have significant effects on plant growth rates, seed production and overall fitness. Considering the high proportion of productivity that is consumed by polyp‐feeding fishes, it would seem incongruous that these grazing fishes do not have similar effects on coral community structure and population dynamics. Our findings highlight the need to revisit previously held assumptions regarding the functional importance of corallivorous fishes to coral reef ecosystems.

Field cage studies were conducted for 2 years to determine the residual toxicities of permethrin, cypermethrin, acephate, methomyl, and several carbaryl formulations against adult Epilachna varivestis Mulsant on soybeans. Tests showed that the residual, wash-off-resistant carbaryl formulations Sevin XLR and UCSF-27 provided control of adult beetles from 14 to 21 days posttreatment. In order of decreasing duration of effectiveness, those two formulations were followed by acephate, permethrin, and cypermethrin, which were active from 7 to 14days; carbaryl (Sevin 80S), active for 7 days; and methomyl, which provided <7 days of control. Although not as active as the residual formulations of carbaryl in causing mortality, permethrin and cypermethrin apparently provided further protection to the soybeans by repelling the adults from the plants past their periods of lethality. This repellency was characterized by low levels of defoliation and by adult beetles avoiding the treated leaves by clinging to the cage or stems.