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Clarifying the amount of pest damage that can be tolerated without justifying insecticidal control will be useful in reducing insecticide use and in development of integrated pest management (IPM) systems. This study investigated the ability of irrigated, high-yielding cotton (Gossypium hirsutum L.) to recover from artificially applied damage simulating that of early season insect pests. Six experiments were done across five cotton-growing seasons. Damage included defoliation, terminal damage, and flower bud removal in a range of timings, combinations, and intensities. Crop yield was unaffected by defoliation applied before first flower buds appeared, even when 100% of true leaves were removed on three occasions (Nodes 2, 4, and 6). Crop maturity was affected by sustained high levels of leaf loss with a peak delay of 10 d after 100% defoliation three times. Up to three light tip damage events or one heavy damage event had no effect on yield and only a slight effect on crop maturity (, COTTON IS ATTACKED by a range of insect pests, some of which are prevalent through the early portion of the growing cycle, such as tobacco thrips (Thrips tabaci Lindeman), tomato [...]

The objective of this study was to assess the ability of cotton (Gossypium hirsutum L.) crops to recover after early-season damage by thrips (Thysanoptera). Such information may help clarify the actual need to control thrips. Ten experiments, resulting from the combination of two to four sites per season and three seasons, were carried out in commercial crops in the irrigation area of northwest New South Wales, Australia. Two treatments were compared: unprotected crops and crops protected with aldicarb{2-methyl-2-(methylthio)propanal O-[(methylamino)carbonyl]oxime} at sowing. Early season thrips communities were dominated by Thrips tabaci Lindeman, which accounted for 52 to 100% of the total phytophagous thrips present in the crops. Insecticide treatment consistently reduced the number of larval thrips compared with the unprotected crops. Number of larval thrips per plant ranged from 0 to 24. Thrips reduced crop leaf area in six experiments, dry matter production in four experiments, and yield in two experiments. Maximum differences in leaf area and dry weight between treatments were found about 40 d after sowing. In all cases, crops damaged by thrips recovered well and reached leaf areas and dry weights similar to protected crops after about 60 to 80 d after sowing. On average, protected crops reached maturity 3 d earlier than crops damaged by thrips, but differences were not statistically significant. Despite large reductions in early growth, yield reductions due to thrips were found in only two experiments. The magnitude of the reduction in yield in those experiments (11%) contrasts with the magnitude of the reduction in growth (about 40%) and highlights the ability of the cotton crop to recover after early season damage by thrips., Young cotton crops host a range of insect pests, including thrips (Thysanoptera) (Hearn and Fitt, 1992). Common species of phytophagous thrips in high-input cotton cropping systems include Thrips tabaci and [...]

Foliar pests can affect reproductive allocation in plants. Depending on bow spider mites (Tetranychus urticae Koch) affect dry matter partitioning in cotton (Gossypium hirsutum L.), yield losses in infested crops could be similar, greater, or lower don expected from growth reduction. In this paper, we analyzed the effects of mite infestations initiated at three stages during the crop cycle on the yield, growth and dry matter partitioning of cotton. Normal- and okra-leaf types, which am known to differ in their responses to mites, were compared in two experiments. Lint yield losses were significant end were greatest for the earliest mute infestations. Reduced Shoot dry matter and reduced harvest index (lint yield/shoot dry matter) both contributed to these yield reductions. Yield losses were less severe in the okra-leaf cultivar than in its normal-leaf counterpart; this was in part due to a more favorable partitioning to key yield components in the former. to both experiments, reduced lint fraction [g lint [(g seed cotton).sup.-1] contributed to the lower harvest index of damaged crops. Reductions in both fruit number per unit shoot dry matter and seed cotton mass per fruit also contributed to reductions in harvest index of damaged crops. Despite the observed reductions in harvest index and some of its components, allometric analysis showed that mites did not affect partitioning to reproductive organs even in the more severely infested treatments. Rather, the low harvest index of mite-infested crops was the result of small plant sm. We conclude that mite effects on cotton growth and yield could be modelled by taking into account their primary effect on radiation-use efficiency and leaf senescence. Explicit modelling of the effects of mites on dry matter partitioning may not be necessary., Spider mites (Acari: Tetranychidae) can severely reduce cotton lint yield (Wilson, 1993, and references cited therein). Mite effects on yield are primarily related to reduction in shoot growth which is, [...]

Two-spotted spider mites Tetranychus urdow Kock) are important pests of cotton (Gossypium hirsutum L.). The effects of mites on cotton photosynthesis have been investigated at the leaf and cytological levels but not at the canopy level. Our objective was to quantify the effects of timing and intensity of infestation by mites on cotton radiation-use efficiency (RUE). Leaf area, light interception, RUE, canopy temperature, and leaf nitrogen concentration (LNC) were assessed during two growing seasons in crops artificially infested with miles between 59 and 127 d after sowing. Normal and okra-leaf cultivars were compared. A mite index (MI = natural log of the area under the curve of mite number vs thermal time) was used to quantify the cumulative effects of mites on RUE, LNC, and canopy temperature. Crop growth reduction due to mites was greater in early- than in late-infested crops. Growth reduction was primarily due to reductions in RUE, but in the more severe treatments accelerated leaf senescence and, henca, reduced light interception also contributed to reductions in crop growth. At a given date, infested okra-leaf crop usually had greater RUE than their normal-leaf counterparts. Both plant types, however, responded similarly to a given level of mite infestation. The ability of the okra-leaf cultivar to maintain greater RUE Levels can be attributed, therefore, to its relative inhospitality for the development of mite colonies rather than to an intrinsically greater capacity to maintain photosynthesis under mite damage. Canopy temperature, LNC, and RUE showed a similar, biphasic pattern of response to MI. In the first phase, response variables were almost unaffected by mites. In the second phase, there was a marked decline in RUE and LNC and a marked increase in canopy temperature with increasing MI. These results suggest (i) some degree of compensatory photosynthesis at low to moderate levels of mite infestation, and (ii) reductions in RUE of mite-infested cotton crops involved alterations in both canopy gas diffusion and metabolic activity. Quantitative relationships between RUE and MI were developed that could be used to link mite and crop growth models., Spider mites (Acari: Tetranychidae) are important secondary pests in cotton cropping systems which rely heavily on use of insecticides for the control of primary pests (Hearn and Fitt, 1992). While [...]

Seed number, the main yield component of cereals and oil-seed species, strongly depends on the physiological status of the crop during a critical period for seed set. Using a comparative approach including three species with contrasting reproductive strategies, we investigated the relationship between seed number per plant (SNP) and plant growth rate during the critical period for seed set ([PGR.sub.C]). Indeterminate soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and maize (Zea mays L.) crops were grown under a wide range of plant densities to generate contrasting availability of resources per plant. Growth of individual plants was estimated by a novel, nondestructive method based on relationships between actual shoot dry matter and morphometric variables, including stem diameter, plant height, and dimensions of reproductive structures. Seed number per plant ranged from 0 to 890 in soybean, 0 to 4096 in sunflower, and 0 to 1348 in maize and [PGR.sub.C] (g [d.sup.-1]) from 0.01 to 4.3 in soybean, 0.3 to 17.6 in sunflower, and 0.4 to 12.3 in maize. Our study showed that (i) the relationship between SNP and [PGR.sub.C] was linear in soybean, reflecting the reproductive plasticity of this species, and curvilinear in sunflower and maize, reflecting morphogenetic restrictions to generate reproductive sinks under favorable growing conditions; (ii) the PGRc threshold below which no seed was set varied among species, being negligible in soybean, close to 0.35 g [d.sup.-1] in sunflower, and 1 g [d.sup.-1] in maize. Quantitative relationships between seed number and plant growth rate during the critical period of seed set could be useful for crop modeling., SEED NUMBER, the main yield component of cereals and oil-seed crops, is strongly dependent on genotype, environmental and management factors (Egli, 1998). In soybean, sunflower, and maize, seed number depends [...]

Part II. Analyzes the effects of mite infestations initiated at three stages during the crop cycle on the yield, growth and dry matter partitioning of cotton. Lower harvest index of damaged crops; Timing of infestation and peak mite numbers of artificially infested cotton crops; Reproductive allocation.

Part I. Quantifies the effects of timing and intensity of infestation by mites on cotton radiation-use efficiency. Leaf area and light interception; Marked decline in radiation-use efficiency; Biphasic pattern of response to the mite index; Alterations in canopy gas diffusion and metabolic activity.