Timing Of Defoliation Research Articles

Leaf number as a criterion for determining defoliation time for Lolium perenne, 1. Effect of water‐soluble carbohydrates and senescence

AbstractThis study sought to define the optimum defoliation interval for Lolium perenne, with the maximum interval being determined by the onset of senescence, as reflected by the number of fully expanded leaves, and the minimum interval set by the replenishment of water‐soluble carbohydrates (WSCs).In a glasshouse set at 13°C/23°C night/day temperature and at a plant spacing of 94 m−2 (3290 tillers m−2), the accumulation of dry matter against leaf number and days was exponential to the four‐leaf stage of the regrowth cycle. Senescence commenced at the 3·5‐leaf stage.WSCs in roots, stubble and leaf showed a very significant linear relationship with leaf number. Levels of WSCs in leaves were restored to predefoliation values by the one‐leaf stage, after which WSCs accumulated preferentially in the stubble, rising to 22% at the four‐leaf stage.Regrowth after 6 d was significantly greater for plants defoliated at the three‐leaf stage than at the one‐leaf stage, but this difference disappeared at the end of the regrowth cycle. This questions the importance of WSCs in determining the yield of ryegrass under normal rotational grazing management.Regrowth at 6 d was related positively to stubble WSCs (r2= 0·66) and to stubble DM per tiller(r2=0·71)

Distance, Rotation, and Border Crops Affect Colorado Potato Beetle (Coleoptera: Chrysomelidae) Colonization and Population Density and Early Blight (Alternaria Solani) Severity in Rotated Potato Fields

The effect of distance between rotated potato fields on Colorado potato beetle, Leptinotarsa decemlineata (Say), and early blight, Alternaria soiani , incidence was evaluated for 2 yr. In eight newly established potato fields, the timing of adult beetle colonization, population densities, and early-season defoliation were related closely to how isolated the fields were from the previous year’s planting. Even short distances between rotated locations resulted in significant reductions of Colorado potato beetle densities. An integrated pest management program resulted in an inverse relationship between distance and the number of insecticides applied for the Colorado potato beetle. Compared with a nonrotated field, a distance of 0.3 to 0.9 km was sufficient to reduce insecticide requirements by 50%. Winter wheat and hay buffers Significantly delayed overwintered adult colonization compared with fallow corn stubble. Early blight severity decreased as the distance between the rotated locations increased. Colorado potato beetle population densities, defoliation, and early blight severity followed a similar exponential decline with distance. The effects of winter wheat and hay buffer crops on beetle infestations and early blight severity were also similar. Factors that reduced early-season Colorado potato beetle immigration also lowered the early blight severity.

552 PB 340 DORMANCY DEVELOPMENT AND STRESS TOLERANCE IN `BRAEBURN' AND `FUJI' APPLE NURSERY STOCK

Studies were performed on the development of dormancy, cold hardiness, and desiccation tolerance, and the effect of manual defoliation timing on performance of `Fuji' and `Braeburn' apple nursery stock. Dormancy development, response to defoliation, and desiccation tolerance of apple differed from those reported for other temperate woody plant species. Dormancy development in `Fuji' was approximately two weeks ahead of `Braeburn', and was strongly regulated by temperature. Photoperiod had no influence on dormancy development of `Fuji'. Desiccation tolerance of both varieties was greatest just prior to the onset of dormancy and early dormancy. This pattern in the seasonal development of tolerance to desiccation is not typical of temperate woody plant species. Early defoliation was detrimental to performance of `Braeburn', but had little effect on `Fuji'. Early defoliation promoted earlier spring budbreak in `Fuji'. Development of freezing tolerance in both apple varieties was typical of other woody plants, and coincided with the onset of dormancy. Maximum hardiness was achieved after the requirements for dormancy were completely satisfied.

Postfire Defoliation Response of Agropyron spicatum and Sitanion hystrix

Prescribed fire is an important management tool to increase herbaceous productivity and maintain seral communities of Intermountain sagebrush grasslands. Currently, pastures treated with prescribed fire are rested from livestock grazing before burning to allow fine fuel accumulation, and rested a full year following burning to restore bunchgrass vigor. This study examined the post-fire defoliation response of Agropyron spicatum and Sitanion hystrix to three different defoliation treatments: an early season defoliation during the boot phenological stage, late season defoliation after seedset, and non-defoliation. Plant response to these defoliation timings was compared to determine the amount of non-use required to restore vigor to these species following fire. Mortality was low for late and non-defoliated plants of both species. Results indicate no differences between late season and non-defoliated plants for biomass production, basal area, or culm number for Agropyron. No differences in Sitanion culm number resulted between the late and non defoliated treatments. Sitanion biomass production differed between late and non-defoliated treatments at Emmett, but not at Craters of the Moon. Basal area and culm number of late and non-defoliated plants of both species increased as compared to the first post-burn season.

Effect of Partial Defoliation on Growth and Carbon Exchange of Two Clones of Young Populus trichocarpa Torr. & Gray

Abstract A controlled environment study was performed to test the effect of two levels of a single partial defoliation on growth, dry weight partitioning, and gas exchange of two contrasting clones of Populus trichocarpa Torr. & Gray. One clone was from a moist habitat in western Washington and the other from a dry habitat in eastern Washington. Trees were partially defoliated (40% or 80% of leaf area removed) at the 25-leaf stage. Height, diameter, leaf area, and rates of net photosynthesis, dark respiration, transpiration, and leaf conductance were determined at the time of defoliation and 3 wk, and 5 wk after defoliation. Biomass distribution was determined following harvest 5 wk after defoliation. Defoliation caused reductions in height, diameter, and biomass in both clones compared to control plants. However, reductions were not proportional to level of defoliation, suggesting compensatory growth was occurring. Total biomass was reduced by 30% and 27% in the western and eastern Washington clones respectively by a 40% defoliation, and 43% and 35% following an 80% defoliation. A 40% defoliation caused an increase in the proportion of biomass in lateral branches of trees of the western Washington clone. An 80% defoliation caused an increase in the proportion of biomass in lateral branches of both clones. Both defoliation treatments reduced leaf area on the main stem but increased leaf area on lateral branches. Both levels of defoliation reduced root biomass of both clones. Specific leaf weight of leaves produced after defoliation in both clones increased. Partial defoliations of 40% and 80% had no effect on net photosynthesis of either clone. Rates of dark respiration in both clones were either unaffected by defoliation or slightly reduced. For. Sci. 39(3):419-431.

Effects of Velvetleaf Competition and Defoliation by Soybean Looper (Lepidoptera: Noctuidae) on Yield of Indeterminate Soybean

The individual and combined effects of various levels and timing of insectinduced defoliation and velvetleaf ( Abutilon theophrasti Medicus) competition on soybean yields were measured during four years of study. Larvae of caged adult soybean loopers, Pseudoplusia includens (Walker), were used to attain planned levels and schedules of defoliation in experimental units consisting of two rows of soybeans 1.8m in length, either weed free or infested with two densities of velvetleaf. Estimates of actual levels of defoliation were recorded from leaflet samples in the first 2 yr of the study, and percentage reduction in leaf area index (LAI) compared to nondefoliated controls was measured in the last 2 yr. Soybean yield was significantly reduced by both defoliation and weed competition, and combinations of defoliation and velvetleaf competition had an additive effect on soybean yield reductions in some plots. Soybean yields were generally lowest in treatments consisting of the highest levels of defoliation and densities of velvetleaf. However, these results showed little, if any, interaction between insect defoliation and velvetleaf competition at the levels included in this study. At soybean growth stage R2, percentage defoliation was a better predictor of yield loss than percentage reduction in LAI, but at R4 the reverse was true.

Effect of Timing and Intensity of First Defoliation on Subsequent Production of 4 Pasture Species

Two simulated pasture studies were carried out at Swift Current, Sask., to determine the effects of date and height of first harvest in the year following establishment on the yield of 4 species in subsequent years. Altai wild ryegrass [Leymus angustus (Trin.) Pilger], Russian wild ryegrass [Psathyrostachys junceus (Fisch.) Nevski] and crested wheatgrass [Agropyron desertorum (Fisch.) Schult.] were used in both studies; alfalfa [Medicago sativa L. ssp. X varia (Martyn) Arcangelil] was included in the second study only. In both studies plots were first cut at 1 of 6 dates, approximately 2 weeks apart, during the period from mid-May to late July in the year following establishment. Cuts were made either at a 5 cm or at a 10 cm height to simulate medium grazing and light grazing, respectively. After the first cut, forage was harvested in that year whenever 10 cm of regrowth was present. In the next 5 years in the first study and 3 years in the second, all forage was harvested at 5 cm height on 15 May and thereafter whenever 10 cm of regrowth was present. Treatment effects were greatest in the year treatments were imposed and in the first post-treatment year. In the treatment year, highest forage yields were obtained from plots cut at 5 cm, but in subsequent years, forage yields were higher from plots cut at 10 cm in the treatment year. Moisture conditions in the seeding and establishment years affected the response to treatments. Date of first cut in the treatment year affected yields in subsequent years in both experiments although differences declined with time. Crested wheatgrass was more affected by date of first cut than were the other species. The cutting dates for highest yields in each case correlated with flowering date and indicate that seedings of these grasses should not be harvested until they have flowered. The results of this study also indicate that to ensure continued high forage yields, the first harvest of these grasses should be less intense than subsequent harvests.

Compensatory Photosynthesis of Sporobolus flexuosus (Thurb.) Rydb. Following Simulated Herbivory in the Northern Chihuahuan Desert

Leaf net photosynthesis (Pn), conductance to water vapor, water potential, solar radiation penetration within the canopy, and biomass production of Sporobolus flexuosus plants in the northern Chihuahuan Desert were measured 4, 18, and 34 d following removal of one-half of all shoots. Regrowth in partially defoliated plants was limited because of a delay in any substantial precipitation until relatively late in the growing season and the mature stage of plant development at the time of defoliation. There were no differences in Pn between partially defoliated and control plants 4 d after defoliation but on days 18 and 34, Pn rates of defoliated plants were 35% higher than for leaves of similar age on control plants. However, significant differences between treatments did not occur until midday when incident solar radiation was greatest. Daily integrated solar radiation penetration within the canopy of defoliated and control plants averaged 66% and 32%, respectively, of the total irradiance received in the o...

Defoliation Effects on Yield and Bud and Tiller Numbers of Two Sandhills Grasses

Intensive grazing strategies for the Nebraska Sandhills must be based on time and frequency of defoliation of key warm-season grasses. A 3-year field study was conducted in the Nebraska Sandhills to determine the effects of defoliation on yield and bud and tiller number of sand bluestem [Andropogon gerardli var. paucipilus (ash) Fern.] and prairie sandreed [Calamovilfa longifolia (Hook.) Scribn.]. Defoliation (7 cm) treatments imposed on a 1.5 X 1-m plot were: a single defoliation on 10 June, 10 July, or 10 August; 2 successive defoliations on 10 June and 10 August; or 3 successive defoliations on 10 June, 10 July, and 10 August. All plots were harvested in October to obtain aftermath yield. Control plots were harvested only at the end of the growing season (October). Defoliation treatments were initiated in 1986, 1987, and 1988 on different plots and the effect of year of initiation as well as the effect of 3 successive years of repeated treatment (1986 plots) was evaluated. Annual dry matter (DM) yield, and bud and tiller numbers were measured. Following the initial year of treatment multiple defoliations increased yield of both grasses while bud and tiller numbers were similar to those of the control plants. After 3 years of repeated treatment, annual DM yield of sand bluestem for all defoliation treatments was lower than the control. A single defoliation of sand bluestem in August or a June-July-August defoliation reduced bud number compared to other treatments and the control. A June-August defoliation of prairie sandreed over a 3-year period increased annual DM yield compared to all treatments and the control although defoliation treatments reduced bud number. The optimum time and frequency of defoliation for annual DM yield and bud and tiller number was a single June or July defoliation for sand bluestem and a June-August defoliation for prairie sandreed.

Effect of Intensity and Timing of Defoliation on Growth, Yield Components and Grain Yield in Maize

SummaryEarly season defoliation of maize significantly reduced plant height and leaf area index but leaf number and leaf emergence rate were not affected. Ear development was significantly affected by both early and late season defoliation although tassel development was barely influenced. Silk growth was the phase of ear development most sensitive to defoliation. Early season defoliation significantly reduced all yield components, leading to yield losses ranging from 54 to 100%. The position of the leaves which were removed had an important effect on the final grain yield. Harvest index was affected by both the intensity and timing of defoliation.

Defoliation Effects on Grain Fill, Stalk Rot, and Lodging of Grain Sorghum

Yield response to defoliation in grain sorghum [Sorghum bicolor (L.) Moench] can be influenced by time and intensity of leaf removal. Limited information is available for defoliation ✕ environment interactions and the defoliation response of yield‐related traits, stalk rot, and basal stalk lodging. This study was conducted to examine the effect of time and extent of defoliation on hybrid grain sorghum in three eastern Nebraska environments in relation to yield components, duration and rate of grain filling, nonsenescence period, stalk rot, and lodging. Four levels of defoliation (0, SO, 95, and 100%) were applied to plants at 10 d preanthesis, anthesis, 10 d post anthesis, and 20 d post anthesis. Defoliation significantly decreased yield components, grain‐fill duration, grain‐fill rate, and the nonsenescence period, while increasing the occurrence of stalk rot. The most pronounced effects on all traits occurred when plants were defoliated prior to or during anthesis. Significant defoliation level ✕ time interactions and environment ✕ defoliation treatment interactions were observed for all traits. Variance component estimates indicated that environmental interaction comprised 18 to 25% of total variation for the yield components, 5% for stalk rot, and 50% for lodging. Use of defoliation in a limited number of environments may not prove effective in evaluating tolerance to lodging. Small environmental interactions for the stalk rot response suggest that observations from only a few environments are necessary for its evaluation.

Effect of Defoliation Time on Depth of Dormancy and Bloom Time for Low-chill Peaches

In a relatively low-chill environment, two cultivars of peach [Prunus persica (L.) Batsch] differing in chilling requirement (`Flordaprince', 150 units and `Flordagold', 325 units) were defoliated at 10-day intervals during midautumn. Effects of defoliation on depth of bud dormancy and dose-dependent responses of cuttings to hydrogen cyanamide throughout the dormant period were analyzed to develop a dormancy index (DI). DI values indicate that early defoliation reduces depth of bud dormancy throughout winter for both cultivars. For `Flordaprince', this was translated into early leafing and bloom, but fruit size was reduced by early relative to late defoliation. In contrast to `Flordaprince', vegetative and floral budbreak of `Flordagold' were delayed by early defoliation. These results indicate that early defoliation affects depth of dormancy and growth ability of buds, but that the extent to which these factors affect bud development depends on cultivar.

Defoliation Effects on Production and Morphological Development of Little Bluestem

Response of key warm-season grasses to time, frequency, and duration of defoliation is needed to develop grazing systems for the Nebraska Sandhills. A 3- year (1986 to 1988) study was conducted on a Valentine fine sand (mixed, mesic Typic Ustipsamments) at the Gudmundsen Sandhills Laboratory near Whitman, Nebraska, to determine the effect of defoliation on little bluestem [Schu 2 defoliations on 10 June and 10 Aug.; and 3 defoliations on 10 June, 10 July, and 10 Aug. Control plants were huvested only at the end of the growing season (October). All plots receiving summer defoliation were harvested in October to obtain aftermath yield. Treatments were initiated in 1986,1987, and 1988 and the effects of l,2, and 3 years of defoliation on dry matter (DM) yield, bud and tiller numbers, and tiller weight were measured. Experimental design was a split block with 4 plants as replications. In the first yeu of treatment annual DM yield from control plants was 2 times greater than that from all defoliated plants, but bud and tiller numbers were similar. In the second year of treatment, all treatments reduced annual DM yield and morphological development below that of the control if precipitation was subnonnal, but not if precipitation was above normal. In the third year of defoliation, with above-nonnd precipitation, single June or July defoliations produced DM yields and morphological development similar to that of the control, but single August or multiple defoliations generally reduced yield md development. Little bluestem may not persist if exposed to multiple, close defolitions during the growing season.

DEFOLIATION EFFECTS ON BIOMASS YIELD COMPONENTS OF WINTER WHEAT

Short duration, high intensity grazing is often used to maximize production of grazed forage crops. Relatively little information is currently available for this practice on dual purpose cereal crops such as wheat. Components of soft white winter wheat biomass yield were studied during the 1979 and 1980 crop years near Corvallis, Oregon. Treatments were a factorial arrangement of two defoliation times (mid-April and mid-May) and three defoliation intensities (undefoliated and clipped to either an 8-cm or 4-cm stubble). All defoliation regimes reduced grain yields compared to undefoliated controls. However, biomass yields of 8-cm vs. 4-cm treatments were similar within defoliation dates. Grain yields primarily reflected the most recent defoliation event, suggesting that plant phenological stage and/or the length of the regrowth period following defoliation was more important than the intensity of the defoliation event in determining plant yield.Key words: Short duration grazing, grazing wheat

Changes in fatty acid composition of sunflower (Helianthus annuus) seeds in response to time of nitrogen application, supply rates and defoliation

AbstractThe fatty acid composition of sunflower (Helianthus annuus L ) seed oil was shown to respond to N supply rates and the time of N application in four glasshouse and two field experiments using five cultivars. Changes in individual fatty acids % total were smaller than those caused by differences in minimum temperatures during seed development but they were statistically significant. N supply rates before floret initiation affected the % fatty acid composition of mature seeds; the percentage of palmitic (16:0) and linoleic (18:2) acids responded positively to increases in N supply whereas % stearic (18:0) and oleic (18:1) acids responded negatively. The only fatty acid to respond to N supply rates between floret initiation and anthesis was 18:0 acid. The percentage of 18:1 responded positively and that of 18:2 acid negatively to high N supply rates after anthesis Responses to single N applications at different stages of plant development in the field confirmed these findings but results were more variable than in the glasshouse. Partial or complete defoliation at anthesis in the field caused the same changes in fatty acid composition as did a low N supply after anthesis.The results are difficult to interpret in terms of our current knowledge of lipid biosynthesis. N supply rates may affect the rate of hydrolysis of fatty acid complexes or their transport from the proplastid to the cytosolic compartment. There was no compelling support for the hypothesis that N supply rates bejore floret initiation were affecting fatty acid composition through their effects on other plant characters such as leaf mass, seed number or single seed weight.

Rootstock and Scion Influence Growth, Productivity, Survival, and Short Life-related Performance of Peach Trees

Abstract Fifty-nine available combinations of 16 peach [Prunus persica (L.) Batsch] seedling rootstocks and four cultivars were evaluated for survival, growth, productivity, and peach tree short life (PTSL) performance of scions for 10 years (1975–1984). Rootstock influenced tree survival, cold and bacterial canker damages, root suckering, bloom date, degree of budbreak, and fruit yield. However, rootstock had little effect on bud density, fruit maturity and size, and time of autumn defoliation, and no influence on trunk circumference and bark gummosis. Cultivars differed in all characteristics except tree survival and canker damage. Tree survival was negatively correlated with budbreak, bloom date, cambial browning, Pseudomonas canker, suckering, and defoliation. Lovell rootstock had the best overall PTSL-related performance, while Siberian C had the worst. ‘Derby’ was the most desirable and ‘Hamlet’ the least of the four cultivars evaluated.

Seasonal Trends for Nonstructural Carbohydrates in Stem Bases of Defoliated Switchgrass

Defoliation management of forage species in the northern USA is related to total nonstructural carbohydrate (TNC) concentrations. These energy compounds are closely related to winter survival, stand persistence, and subsequent productivity of stands. Influences of early spring defoliation on TNC concentrations of switchgrass (Panicum virgatum L.) during recovery growth in the humid USA have not been reported. This field study was conducted to observe influences of defoliation time and intensity on TNC concentrations during recovery growth when switchgrass was grown without or with N fertilization (120 kg N ha−1). The experimental site was a Webster‐Nicollet soil complex (fine‐loamy, mixed, mesic Typic Haplaquolls‐Aquic Hapludolls). Defoliation treatments at 2‐wk intervals in 1984 and 1985 consisted of early‐, mid‐, and late‐spring defoliation, each at moderate or severe intensity. Nitrogen fertilization reduced TNC concentrations for all defoliation treatments. Accumulated TNC concentrations remained lower after 3, 6, and 9 wk of recovery growth for 120 compared with 0 kg N ha−1, but relatively little difference was observed among defoliation treatments in TNC concentrations by mid‐October. Severe defoliation at early, mid, and late spring generally contributed to lower TNC concentrations during recovery growth compared with moderate defoliation. However, TNC concentrations 6 to 8 wk or less after spring defoliation were at a relatively high concentration for most defoliation treatments, which suggests that herbage utilization in mid‐ to late‐July should not seriously weaken stands. Lower TNC concentrations in mid‐October of 1985 compared with 1984 suggest that both weather conditions and defoliation management influence TNC concentrations in stem bases of switchgrass.

The effect of differences in intensity and frequency of defoliation on the growth of Sirolan phalaris in the field

A two-year old sward of Phalaris aquatica cultivar Sirolan was cut at 2-, 4-, 6- or 12-week intervals and at heights of 2 or 7 cm over two 12-week cycles from June to November 1986. Defoliation treatments were split and 100 kg ha-1 of nitrogen was applied to half the plot at the beginning of each cycle. Tiller density and dry matter production were measured throughout winter and spring of 1986, and in two 6-week regrowth periods in 1987.During winter, high cutting always gave less harvested dry matter than low cutting. In spring, the regression lines relating cutting frequency to cumulative harvested dry matter crossed over at a frequency of about 5 weeks. Below this frequency high cutting yielded more harvested dry matter than low cutting. High cutting gradually changed sward appearance in winter, resulting in better yields at later harvests. At the conclusion of spring cutting, there were more dead tillers on plots cut at 6- and 12-week intervals than on those cut every 2 or 4 weeks, while numbers of live tillers increased progressively as cutting interval rose from 2 to 12 weeks. Regrowth in autumn 1987 was much better on plots cut every 12 weeks than on other plots, primarily owing to the greater vigour of individual tillers, but also owing to higher tiller numbers. The timing of defoliation in spring appeared to be the most important factor governing crown bud production and regrowth in the following season. Spelling in spring is recommended to permit reproductive and crown bud development in Sirolan.

The effects of the spatial pattern of defoliation on regrowth of a tussock grass : I. Growth responses.

The effects of the spatial pattern of defoliation within a tussock grass, Agropyron desertorum, were investigated at a semiarid field site. In the middle of the spring growing season (mid-May), tussocks were clipped in repeatable defoliation patterns, and the regrowth of foliage was monitored. These clipping patterns involved removal of foliage from different locations within the tussock, but the total amount of foliage removed was held constant. Active meristems were left intact in all cases. The spatial pattern of defoliation affected both initial rates of tussock regrowth and total growing-season aboveground biomass production. When leaves were removed low in the tussock (older leaves), regrowth was greater than after removal of the same quantity of foliage high in the canopy (younger leaves). These differences in regrowth were due to differences in the rate of new tissue production rather than differences in the timing of senescence. The results were consistent over two years even though aboveground production differed considerably between years. The interaction of the spatial pattern and timing of defoliation was also studied by clipping additional plants in late May. The timing of defoliation affected the relative influence of different defoliation patterns on regrowth. In those defoliation patterns where active meristems were not removed in the late-May clipping, there were no differences in regrowth of tussocks which had either upper or lower foliage removed. However, because the grass culms had elongated by late May, active meristems were higher and were removed by one of the defoliation pattern treatments (a uniform clipping). This resulted in much less regrowth. Differences in the effects of clipping patterns applied in late May were associated with this removal of active meristems. Whereas, differences among clipping treatments following the earlier mid-May defoliation were probably a result of changes in factors which affected tussock carbon gain (e.g., light interception, foliage age structure). In either case, the spatial pattern of defoliation within a tussock grass clearly had significant effects on aboveground regrowth.

Annual Life Cycles of Two Populations of Eotetranychus tiliarium (HERMANN) (Acarina : Tetranychidae) on Different Alder Trees

The annual life cycle and diapause characteristics of Eotetranychus tiliarium (HERMANN) were studied in two Sapporo populations on alders (Alnus japonica and A. hirsuta). Over-wintered females appeared on leaves in early May (A. japonica population) and in late April or early May (A. hirsuta population), and were observed on leaves until early November and late October to early November on their respective hosts. It was estimated that there were probably 5 generations a year in both populations. In the A. hirsuta populations, all stages of mites were seen on leaves until the end of October, but this was not true in the A. japonica population. The photoperiodic responses of the two populations were similar and the proportion of diapausing females reached 100% at 13.5 hr of light or less at 18°C. The critical photoperiods in both populations were abotu 14.0 hr. Based on the diapausing status of those collected from the field, all the females of both populations should have entered diapause in October, and thus survival of all stages of the A. hirsuta population in late October seemed unexplainable. These discrepancies between stage structure and diapause response were discussed from the viewpoint of differences in the host plant trait at defoliation time and the mode of life of the mites. Some females of both populations, collected from bark crevices in early October, began to oviposit when incubated at 25°C and 15L-9D. The number of days required for reactivation gradually decreased towards April, while the proportion of laying females, on the contrary, increased.