Johnson, Marshall W.

LaPr, Larry F. : Larry F. LaPr is a private environmental consultant, Department of Entomology, U.C., Riverside.

Oatman, Earl R. : Earl R. Oatman is Professor of Entomology and Entomologist, Department of Entomology, U.C., Riverside.

Sances, Frank V. : Frank V. Sances, former Post-graduate Research Assistant, University of California, Riverside, is now head of Pacific Agricultural Laboratories, San Diego

Toscano, Nick

Day - degreerequirements for development of life stages of San Jose scale * Fi.eld data indicatethat natural populations Day - degrees of the two - spotted spider mite , Tetranychus Stage Male Female urticae Koch , can decrease strawberry yields When we evaluated a so - called mid - curve , by 10 to 15 percent at various harvest periods Embryo ( egg ) development 405 421 in which all stages ( black caps , mature 1964 , entomologists during the season . Since mated , and unmated females ) of San Jose at University of California , Riverside , have First instar scale would be expected to survivethe winter been conducting research to develop a prac - ( crawlers , white caps , and produce individualsin the spring genera - 318 331 black caps ) tical program for managing the mite on tion , we found a much closer correlation summer - and winter - planted strawberries in Second instar 213 220 between observed events and what the model of southern California , studyingthe efficacy projected should happen . This was true for predatory mites and selective miticides , as Pupal stages - males 95 both time periods : from January 1 until first well as the injury caused by spider mite Tight cap - females 58 males were trapped , and from first males Feeding . Effectivecontrol measures are being Pre - matingadults 19 20 trapped until first crawlers were observed . sought for incorporation into integrated pro - When the third hypothetical generation or yams in commercial strawberry plantings . Average DOIgeneration 1050 1050 growth curve was evaluated , in which only * Lower threshold = 51 F ; upper threshold = 90 ' F . The Photosynthesisand productivity overwintered mature females would be ex - lower threshold is the temperature below which the insect's development stops ; the upper threshold the temperature pected to survive , we again found consider - In California , commercial strawberry vari - above which the development rate begins to decrease . The amount of heat between the two thresholds that is needed able variation between observed events in the eties produce fruit continuously throughout for the insect to develop from one stage to the next is field and those the model projected , particu - the winter and spring . Fruit filling and calculated in daydegrees - the degrees of temperature above a threshold for each day . Doaccumulationsfrom first males larlyin the enlargement occurs when plant canopies are until first crawlers appeared in the field . established and after the plant's initial stor - alternative to laborious inspection of fruit From a practical standpoint for California , age reserves have been utilized . Carbohy - the early or mid - curves would be the most drates and other materials migrating into or the even less desirable method of wood finding infested fruit . After the pheromone realistic with regard to scale development . developing berriesoriginatepredominantlyin trapping system for San Jose scale was in - In considering how the San Jose scale chlorophyllous tissues , and the relativequan - troduced , research cooperators in Utah and of nutrients available primarily depends model can be used in pest management pro - tity on the rate of photosynthesis , in which sugars Michigan produced a phenology model for grams , application of chemical controls for ( photosynthates ) are produced by the bind - scale , using a large amount of California scale crawlers in May would be one of the ing data . Early - season ( first - flight ) trapping of carbon and water molecules into most critical areas to evaluate . Based on carbon - based chains . Photosynthesis occurs model projections , we would anticipate data used in conjunction with the model within the chloroplasts of leaf cells and is should lead to improvedtiming crawler emergence approximately of treatments 400 Do powered by solar energyabsorbed by the pig - after the first males in any given generation for San Jose scale , particularly against ment chlorophyll . The required water is crawlers in late April or early May . Using have been collected in pheromone traps . As brought up from the roots through the pro - in the case with oriental fruit moth egg hatch , of 51 O and lower and upper threshold values of translocation . The second compon - cess however , we suspect that spray timings at the 90O F , respectively , the day - degree require - ent , carbon dioxide , enters the leaf tissue of crawler emergence prob - very beginning ments as used in the model are given in the through the stomata - small closable aper - ably would not be optimum , but instead table . tures in the epidermis . While stomata facili - The model accommodates three hypothe - should be delayed for several days . This tate movement of carbon dioxide into the means that we could add approximately tical types of growth curves for San Jose 100 leaf , they also allow large quantities of water scale . In the first cycle or generation curve to 150 Doto the crawler emergence curve to be lost by evaporation to the atmosphere - that we might expect to see for San Jose scale , beyond first crawler and spray at that point . a process known as transpiration . When This approach to spraytiming for San Jose only overwintered black caps ( late first - stage transpiration rates exceed the rate of water scale ) would be producing new crawlersin the scaleneeds to be verified with field plot work . uptake by the roots , the plant wilts . However , as in the case with the oriental fruit spring . This is the so - called early curve . Photosynthates are required for respira - 225 moth model , we still feel that the model is Calculations show that , on the average , of vegetative structures tion and growth sufficiently accurate to begin using it under a Dowere required for scaledevelopmentfrom ( leaves , stems and roots ) as well as for fruit variety of pest control advisorand grower ap - January 1 to the point at which first males production . By continuously producing new plications to challenge its validity and detect were observed in pheromone traps , with an foliage , the plant replacesleaves that may die early population curve . However , the model the weak points that undoubtedly exist . within several weeks after development . 274 Do are required , on the projects that Feeding by two - spotted spider mite causes average . Similarly , development from first Richard E . Rice , Entomologist , Department of plant stress , which detrimentally affects male flight in the spring until first crawlers Entomology , Davis , is located at the San Joaquin photosynthesis , transpiration , fruit produc - 323 were observed required approximately ValleyResearch Center , Parlier ; Donald L . Flaherty is Farm Advisor , Tulare County ; and Richard A . tion , and vegetative growth . Reductions in D O . The model projects a day - degreerequire - of Jones , Staff Research AssociateIV , Department photosynthesisrates from mite - caused stress of 372 " . Therefore , in the hypothetical ment Entomology , is located at Parlier . Appreciation is decrease berry production ( fig . 1 ) . The early curve we see a rather wide divergence expressed to the many CooperativeExtensionper - sonnel who have assisted in developing scale of stress and time of the season when amount between observed events ( first male , first trapping data and techniques over the past few stress occurs largely determine the extent of crawler ) and what the model predicts for years . This research was supported in part by injury to the strawberry plant . Ideally , grantsfrom the California Tree Fruit Agreement . these same events . 14 CALIFORNIAGRICULTURE , JANUARY - FEBRUARY 1982 Spider mites can reduce strawberry yields Frank V . Sances 0 Nick C . Toscano 0 Larry F . LaPre ' 0 Earl R . Oatman 0 Marshall W . Johnson Strawberryph t s are particularly susceptible to rapid spider mite buildup early in the semon . The resulting stress reduces quantity and quality of harvestablefmit . Injury caused by high early - seasonpopula - Y = 1.10 + 0.838 ~ tions was more apparent both physiologically TRANSPIRATION r = 0.82 125 - and visually in the field . Distorted , stunted am Low spider mite 20 plant growth was evident , and many mature population as 100 % ! * a - L . _ leaves appeared bronzed and dry on their 2 + lower surfaces . & 15 - 75 - spider mite a population Reduced physiological activity persists in c $ = Early - season ' & - - - plants with high spider mite populations t unless mite densities decline . After mite - E . 10 - Q " " ' I 1 I I W - caused stress decreases , new foliageis rapidly 0 PHOTOSYNTHESIS - initiated . Although such leaves may be small > f and chlorotic , their photosynthesis rates are 5 - comparable to those in older leaves from y , plants on which mite populations have been \ I I suppressed . This host recovery phenomenon recover period does not occur on plants with low , suppressed I mite populations where new foliage is pro - 315 3 / 23 4 6 4 / 20 5,4 5 / 18 6 ) l 6 / 15 612 : duced uniformly throughout the season . Stress - inducingmite population levels sig - Fig . 2 . Seasonal transpiration and photosyn - nificantly influence the number and size of straw - thesis trends of summer - planted fruit produced per plant ( see table ) . De - growers on a sound pest management pro - berries stressed by two - spotted spider mites . creasesin fruit number were greatest in plants gram , using an economic threshold , would Data are plant means , expressed as percent - with high early - season populations . Early - permit somereduction in photosynthesis , but age of rates obtained from nonstressed plants , Irvine , California , 1979 . season mite populations caused earlier and would begin control measures before fruit production is affected if spider mite popula - tions continue to increase . Summer - and wintermplanted berries Studies conducted at the University of California's South Coast Field Station , Santa Ana , have shown that the intensity of straw - berry plant responses to spider mite feeding stressvaries , depending on crop maturity and planting time . Small - plot experiments on summer - planted berries revealed that high spider mite populations late in the season significantly reduced photosynthesis and 2 ) . transpirationratesfrominfestedplants ( fig . Photosynthesis and transpiration rates of plants artificially inoculated with high early - seasonmite infestationsdecreased four weeks earlier than those of plants with high late - season populations . CALIFORNIAAGRICULTURE , JANUARY - FEBRUARY 1982 15 weight from stressed plants decreased , fol - more pronounced effects on berry size than lowed by fruit size , fruit number , flower in - naturally occurring , later seasonpopulations . itiation , and new leaf development . Fruit number and size determine total yield , and changes in either component can Summary influence the overall yield per plant . Plants 140 ( a ) F " ' - I Photosynthesis was negatively correlated naturally infested with high mite populations with accumulated mite - days / leaflet . Under showed a reduction in total yield beginning in of stress , reductions the resulting high levels mid - May . Those stressed by early - season in quantity and quality of harvestable fruit spider mite populations showed significant occurred . Further stressreduced the develop - reductions in total yield one week earlier . ment of flowers and new leaves . Once they began , total yield reductions per - - Transoiration \ ; ' _ _ _ Photc Impaired carbon dioxideuptake ( indicated sisted throughout the remaining season . by reduced transpiration rates ) and fixation Before reductions , all stressed plants exhib - ( synthesisof carbon - based sugars ) reducethe ited increases in total yield weight because of of simplesugarsand other photo - availability the increasein fruit number harvested earlyin synthates required in the development of the season . reproductive and vegetative plant structures . In small - plot experiments , responses of - o - - 4c If mite - day accumulations are gradual , the winter - planted berries to spider mite stress plant compensatesfor a period were similar to those of summer - planted ber - of time before 4 100 % - ries . Photosynthesis and transpiration rates economic injury results . Conversely , when 50 percent lower in plants of plant stress develop rapidly , were as much as high levels wth high late - season populations than in photosynthesis and productivity are severely plants where mites were suppressed ( fig . 3a ) . restricted . Photosynthesis and transpiration reductions Although plants may begin to produce new I foliage during this time , the diminished sup - in stressed plants were accompanied by decreasesin fruit weight , number , and size , in of photosynthates from heavily infested ply leaves and the strong competition for these that order ( fig . 3b ) . Reductionsin photosyn - products by developing fruit may reduce the thesis caused by spider mite feeding also compensation capacity of flowers and new of plants damaged affected the production leaves ( fig . 3c ) . early in the season . Additionally , younger plants have less leaf area to withstand the in - Sequenced mite - day models jury and reduced photosynthates and pro - ductivity occur at much lower mite - day Converting weekly spider mite densities to 26 31 4 9 14 19 24 29 4 9 I4 19 24 29 accumulated mite - days ( one mite - day is levels , when compared with plants that have March April May equivalent to one mite feeding for one day ) gradually increasing infestations of spider Fig . 3 . Plant responsesto mite - caused stress mites occurring after maturation is more ad - makes it possible to express both density and ( a , b , c ) , and cumulative mite - days from duration of a spider mite infestation as one vanced . Plants that undergo a gradual mite - stressed and unstressed plants with 10 per - cent reductions in physiological and growth value . The sequenced mite - day model based day buildup and late - season stressshow more processes chronologically indicated . ( d ) . than twice the tolerance to the samemite - day on data from summer - planted berries ( fig . 4 ) includes only statistically significant events levels that caused injury early in the season . * - - - - - Because of plants ' susceptibility to spider occurring at pertinent points on the popula - , ' Reduction in mite injury early in the season , the necessity tion curves . High early - season populations , ' total yield / weightlplant of the two - spotted spider mite significantly of frequent samplingand pesticideuse would andnumber be greatest during this time . Early - season reduced photosynthesis at 3,200 mite - days control would allow for the production of per leaflet , total yield per plant at 7,600mite - A healthy , well - estab - maximum leaf area . days per leaflet , and each yield component at lished plant canopy with large leaf area not 9,900 mite - days per leaflet . only is more tolerant to spider mite stress , but Although small increases in number of also enables plants to yield higher quality fruit were recorded before the reduction in a tolerant ap - fruit . When plants are mature , photosynthesis in early - season - stressed proach to spider mite infestations is possible . plants , this trend was significant at 1,700 of biological control agents ( preda - The use mite - days on plants subjected to a more tory mites ) may be effective in mid - to late - gradual mite - day accumulation . Photosyn - season , when economic risk is minimal . thesis in plants exposed to high mite - day levels later in the season significantlydeclined only after 7,100 mite - days . Yield was subse - quently reduced after 9,800 mite - days per Frank K Sances , former Post - graduateResearch leaflet . Assistant , University of California , Riverside , is now head of Pacific Agricultural Laboratories , - Low POPULATION A similar sequenced mite - day model based 1 - - - - I_ ~ : , - i $ ARiCl * DE ; SUPEgED ) ; _ - San Diego ; Nick C . Toscano is Entomologist , Co - on data from winter - planted berries ( fig . 3d ) 2 9 16 23 33 6 1 1 23 17 0 4 1 ! e Is I 8 15 22 % U.C . , Riverside ; Larry E operative Extension , includes pertinent points on the population Lapre / is a private environmental consultant ; and June March April May is Professor of Entomology and Earl R . Oatman curveswherephysiological , yield , and growth Fig . 4 . Sequenced mite - day model of early - Entomologist , and Marshall W . Johnson is Post - 10 Percent . Parameters were each ~ duced and late - season spider mite buildups and re - graduate Research Assistant , Department of Ento - sponses of summer - planted strawberries . First , photosynthesis , transpiration , and fruit U.C . , Riverside . mology , 16 CALIFORNIA AGRICULTURE , JANUARY - FEBRUARY 1982