experiments were conducted over three growing seasons ( 20142016 ) in a bartlett pear orchard located at the usda - ars experimental farm near moxee , wa . the orchard was planted in 2001 with 5-m    5-m spacing . the orchard was divided into six plots , each separated by a row of untreated trees . each plot included four experimental trees separated by at least one untreated tree , and each tree within a block was randomly assigned a foliar treatment : employ , actigard , odc , or untreated control . trees received the same treatment assignments each of the 3  yrs of the study . foliar treatments were applied according to the product labels ( table 1 ) using a gas - powered backpack sprayer ( model sr420 , stihl inc . , spray volumes were estimated using spray times with a sprayer rate of 32  ml / second . treatments were applied on 10 april , 8 may , 5 june , and 3 july . in 2015 , treatments were applied on 16 april , 16 may , 11 june , and 9 july . in 2016 , treatments were applied on 14 april , 12 may , 9 june , and 7 july . foliar application rates of defense elicitorstreatmentproduct label rate product/0.4  hamix rateapplication volumecontroln / awater1  l / treeactigard 50wg9.3 to 28.4  g1.2  g/10.7  l water890  ml / treeemploy56.7 to 113.4  g6.9  g/6.2  l water445  ml / treeodc100 to 500  ml8.8  ml/13.3  l water2.2  l / tree1commercial application rates according to the manufacturer 's label.2product mixtures for treatment of individual experimental trees based on the maxium product label rate.3volume of mixed product applied to each individual experimental tree . foliar application rates of defense elicitors commercial application rates according to the manufacturer 's label . product mixtures for treatment of individual experimental trees based on the maxium product label rate . populations of c. pyricola were monitored weekly beginning 1 wk before the first application of defense elicitors and ending in mid - august consistent with harvest dates for bartlett in washington . nymphs were sampled in situ by counting the number of nymphs on the 10 most terminal leaves on one shoot from each of the four cardinal directions around each tree ( four shoots per tree ) . adult psyllids were sampled by tapping a branch from each of the four cardinal directions three times with a 0.5-m - long radiator hose to dislodge the insects onto a 0.2-m white beat tray . as for nymphs , this provided four samples per tree . statistical analyses comparing c. pyricola nymphal or adult counts among treatments were performed using repeated measures ( proc glimmix , sas institute 2013 ) . data collected in 2014 , 2015 , and 2016 were analyzed separately with the number of nymphs or the number of adults per tree ( sum of four sampling units per tree ) as the dependent variables . week , treatment , and the week - by - treatment interaction were included as the fixed effects . week and block were included as the random variables . based on fit statistics and normal and quantile quantile plots , analyses were performed assuming negative binomial distributions using the dist  =  nb option of the model statement ( gbur et al . 2012 ) . an autoregressive covariance structure was specified using the type  =  ar(1 ) option of the random statement . where differences among treatments were indicated , mean number of insects on trees treated with each elicitor were compared with those on control trees using paired contrast statements ( diff  =  control option of the lsmeans statement ) . experiments were conducted over three growing seasons ( 20142016 ) in a bartlett pear orchard located at the usda - ars experimental farm near moxee , wa . the orchard was planted in 2001 with 5-m    5-m spacing . the orchard was divided into six plots , each separated by a row of untreated trees . each plot included four experimental trees separated by at least one untreated tree , and each tree within a block was randomly assigned a foliar treatment : employ , actigard , odc , or untreated control . trees received the same treatment assignments each of the 3  yrs of the study . foliar treatments were applied according to the product labels ( table 1 ) using a gas - powered backpack sprayer ( model sr420 , stihl inc . , spray volumes were estimated using spray times with a sprayer rate of 32  ml / second . treatments were applied on 10 april , 8 may , 5 june , and 3 july . in 2015 , treatments were applied on 16 april , 16 may , 11 june , and 9 july . in 2016 , treatments were applied on 14 april , 12 may , 9 june , and 7 july . foliar application rates of defense elicitorstreatmentproduct label rate product/0.4  hamix rateapplication volumecontroln / awater1  l / treeactigard 50wg9.3 to 28.4  g1.2  g/10.7  l water890  ml / treeemploy56.7 to 113.4  g6.9  g/6.2  l water445  ml / treeodc100 to 500  ml8.8  ml/13.3  l water2.2  l / tree1commercial application rates according to the manufacturer 's label.2product mixtures for treatment of individual experimental trees based on the maxium product label rate.3volume of mixed product applied to each individual experimental tree . foliar application rates of defense elicitors commercial application rates according to the manufacturer 's label . product mixtures for treatment of individual experimental trees based on the maxium product label rate . populations of c. pyricola were monitored weekly beginning 1 wk before the first application of defense elicitors and ending in mid - august consistent with harvest dates for bartlett in washington . nymphs were sampled in situ by counting the number of nymphs on the 10 most terminal leaves on one shoot from each of the four cardinal directions around each tree ( four shoots per tree ) . adult psyllids were sampled by tapping a branch from each of the four cardinal directions three times with a 0.5-m - long radiator hose to dislodge the insects onto a 0.2-m white beat tray . as for nymphs , this provided four samples per tree . statistical analyses comparing c. pyricola nymphal or adult counts among treatments were performed using repeated measures ( proc glimmix , sas institute 2013 ) . data collected in 2014 , 2015 , and 2016 were analyzed separately with the number of nymphs or the number of adults per tree ( sum of four sampling units per tree ) as the dependent variables . week , treatment , and the week - by - treatment interaction were included as the fixed effects . week and block were included as the random variables . based on fit statistics and normal and quantile quantile plots , analyses were performed assuming negative binomial distributions using the dist  =  nb option of the model statement ( gbur et al . 2012 ) . an autoregressive covariance structure was specified using the type  =  ar(1 ) option of the random statement . where differences among treatments were indicated , mean number of insects on trees treated with each elicitor were compared with those on control trees using paired contrast statements ( diff  =  control option of the lsmeans statement ) . experiments were conducted over three growing seasons ( 20142016 ) in a bartlett pear orchard located at the usda - ars experimental farm near moxee , wa . the orchard was planted in 2001 with 5-m    5-m spacing . the orchard was divided into six plots , each separated by a row of untreated trees . each plot included four experimental trees separated by at least one untreated tree , and each tree within a block was randomly assigned a foliar treatment : employ , actigard , odc , or untreated control . trees received the same treatment assignments each of the 3  yrs of the study . foliar treatments were applied according to the product labels ( table 1 ) using a gas - powered backpack sprayer ( model sr420 , stihl inc . , spray volumes were estimated using spray times with a sprayer rate of 32  ml / second . treatments were applied on 10 april , 8 may , 5 june , and 3 july . in 2015 , treatments were applied on 16 april , 16 may , 11 june , and 9 july . in 2016 , treatments were applied on 14 april , 12 may , 9 june , and 7 july . foliar application rates of defense elicitorstreatmentproduct label rate product/0.4  hamix rateapplication volumecontroln / awater1  l / treeactigard 50wg9.3 to 28.4  g1.2  g/10.7  l water890  ml / treeemploy56.7 to 113.4  g6.9  g/6.2  l water445  ml / treeodc100 to 500  ml8.8  ml/13.3  l water2.2  l / tree1commercial application rates according to the manufacturer 's label.2product mixtures for treatment of individual experimental trees based on the maxium product label rate.3volume of mixed product applied to each individual experimental tree . foliar application rates of defense elicitors commercial application rates according to the manufacturer 's label . product mixtures for treatment of individual experimental trees based on the maxium product label rate . populations of c. pyricola were monitored weekly beginning 1 wk before the first application of defense elicitors and ending in mid - august consistent with harvest dates for bartlett in washington . nymphs were sampled in situ by counting the number of nymphs on the 10 most terminal leaves on one shoot from each of the four cardinal directions around each tree ( four shoots per tree ) . adult psyllids were sampled by tapping a branch from each of the four cardinal directions three times with a 0.5-m - long radiator hose to dislodge the insects onto a 0.2-m white beat tray . as for nymphs , this provided four samples per tree . statistical analyses comparing c. pyricola nymphal or adult counts among treatments were performed using repeated measures ( proc glimmix , sas institute 2013 ) . data collected in 2014 , 2015 , and 2016 were analyzed separately with the number of nymphs or the number of adults per tree ( sum of four sampling units per tree ) as the dependent variables . week , treatment , and the week - by - treatment interaction were included as the fixed effects . week and block were included as the random variables . based on fit statistics and normal and quantile quantile plots , analyses were performed assuming negative binomial distributions using the dist  =  nb option of the model statement ( gbur et al . an autoregressive covariance structure was specified using the type  =  ar(1 ) option of the random statement . where differences among treatments were indicated , mean number of insects on trees treated with each elicitor were compared with those on control trees using paired contrast statements ( diff  =  control option of the lsmeans statement ) . repeated - measures analyses did not reveal significant week - by - treatment interactions for either nymphs or adults regardless of sampling year ( table 2 ) , but counts of nymphs and adults varied by sampling week within each year ( table 2 ) . nymphal populations exhibited two generation peaks , which occurred in late april to early may and in june of each year ( figs . the second generation of nymphs was three to four times larger than the first generation in all 3 yrs ( figs . adult populations also exhibited two generation peaks , which occurred about 2 wks after observed peaks in nymphal populations ( figs . the second generation of adults was numerically larger than the first generation , but the second generation was small compared to the first generation in both 2015 and 2016 ( figs . table 2.statistical analyses examining the effects of foliar applications of defense elicitors on c. pyricola populationsvariable201420152016nymphs  weekf18 , 72=40.6 ; p<0.001f16 , 64=24.9 ; p<0.001f18 , 72=32.6 ; p<0.001  treatmentf3 , 12=5.7 ; p=0.013f3 , 12=1.6 ; p=0.253f3 , 12=4.8 ; p=0.020  week  treatmentf54 , 216=1.4 ; p=0.056f48 , 192=0.8 ; p=0.818f54 , 216=1.1 ; p=0.295adults  weekf18 , 72=58.1 ; p<0.001f16 , 64=43.8 ; p<0.001f18 , 72=7.3 ; p<0.001  treatmentf3 , 12=6.3 ; p=0.008f3 , 12=0.4 ; p=0.763f3 , 12=2.6 ; p=0.097  week  treatmentf54 , 216=0.8 ; p=0.806f48 , 192=0.7 ; p=0.900f54 , 216=0.9 ; p=0.643 statistical analyses examining the effects of foliar applications of defense elicitors on c. pyricola populations analyses revealed significant differences in numbers of nymphs among foliar treatments in 2014 and 2016 , but not in 2015 ( table 2 ) . the lack of significant week - by - treatment interactions indicated that the effects of foliar applications on nymphs were similar among weeks . in 2014 , significantly fewer nymphs were observed on trees treated with actigard , employ , or odc than on untreated controls pooled over all sampling dates ( fig . although not statistically significant in 2015 , nearly 2030% fewer nymphs were observed on trees treated with actigard , employ , or odc than on untreated controls ( fig . <  0.1 ) of nymphs on trees treated with actigard in 2015 compared with untreated controls ( t  =  2.1 ; df  =  1 , 192 ; p  =  0.058 ; fig . 3a , right panel ) were recorded from trees treated with actigard , employ , or odc than on untreated controls in 2016 . overall , results from the three sampling years were consistent and suggested that treating trees with defense elicitors leads to a modest reduction in populations of c. pyricola nymphs . these reductions were most obvious during the second - generation population peak , when numbers of nymphs were 2030% greater on untreated trees than on trees treated with elicitors . results of these field trials were also consistent with our previous laboratory study , which indicated that treating pear with foliar applications of actigard- , employ- , or odc - induced systemic defenses that increased mortality of psyllid nymphs ( cooper and horton 2015 ) . 1.mean number of c. pyricola nymphs per shoot ( a ) and adults per beat sheet sample ( b ) in 2014 . dates provided on the x - axis indicate days on which foliar applications were applied . error bars denote ses and an asterisks indicate that values are significantly different from the untreated control treatment.fig . 2.mean number of c. pyricola nymphs per shoot ( a ) and adults per beat sheet sample ( b ) in 2015 . dates provided on the x - axis indicate days on which foliar applications were applied . error bars denote ses and an asterisks indicate that values are significantly different from the untreated control treatment.fig . 3.mean number of c. pyricola nymphs per shoot ( a ) and adults per beat sheet sample ( b ) in 2016 . dates provided on the x - axis indicate days on which foliar applications were applied . error bars denote ses and an asterisks indicate that values are significantly different from the untreated control treatment . mean number of c. pyricola nymphs per shoot ( a ) and adults per beat sheet sample ( b ) in 2014 . dates provided on the x - axis indicate days on which foliar applications were applied . error bars denote ses and an asterisks indicate that values are significantly different from the untreated control treatment . mean number of c. pyricola nymphs per shoot ( a ) and adults per beat sheet sample ( b ) in 2015 . dates provided on the x - axis indicate days on which foliar applications were applied . error bars denote ses and an asterisks indicate that values are significantly different from the untreated control treatment . mean number of c. pyricola nymphs per shoot ( a ) and adults per beat sheet sample ( b ) in 2016 . . figures on the right show the overall effects of treatment regardless of sampling week . error bars denote ses and an asterisks indicate that values are significantly different from the untreated control treatment . significantly more adults were collected from untreated trees than from trees treated with actigard , employ , or odc pooled over all sampling weeks in 2014 ( fig . as observed for nymphs , the differences in treatments were most obvious during the second - generation peak , which occurred in july 2014 ( fig . 3b ) , when the second generation was substantially reduced compared to that of 2014 . although the overall treatment effect was not significant at   =  0.05 in 2016 ( table 2 ; p  =  0.097 ) , paired contrasts indicated that significantly fewer adults were collected from trees treated with employ than from untreated trees ( t  =  2.67 ; p  =  0.020 ) , and marginally fewer adults were collected from trees treated with actigard than from untreated trees ( t  =  1.96 ; df  =  1 , 192 ; p  =  0.073 ) ( fig . our previous laboratory study did not indicate that defense elicitors led to decreased adult survival , but adults did tend to settle and oviposit on untreated trees more often than on trees treated with actigard , employ , or odc in choice assays ( cooper and horton 2015 ) . it is possible that differences among treatments observed in 2014 were due to reduced numbers of nymphs developing to adults on treated trees , and due to movement of adults to adjacent untreated trees . it is unclear whether treatment differences attributed to adult preference would be replicated if an entire orchard were treated with an elicitor product . results of our study demonstrate that foliar applications of actigard , employ , or odc may reduce c. pyricola nymphs under field conditions . these results are consistent with those of our previous laboratory bioassays ( cooper and horton 2015 ) , and with other reports that elicitors of salicylic acid - dependent defenses reduce performance of other phloem - feeding insects ( dong et al . the modest reduction in c. pyricola nymphs observed in our study does not warrant the use of elicitors alone for the control of psyllids . however , elicitors are often used to manage fire blight and knowledge that these elicitors may also partially suppress c. pyricola populations could be useful for system - wide integrated pest management approaches . more trials are required to evaluate the efficacy of these products applied to entire orchards and used in a spray schedule typical for fire blight management .
abstractdefense elicitors are products that activate acquired defense responses in plants , thus rendering the plants less susceptible to attack by a broad range of pests . 
 we demonstrated previously under laboratory conditions that foliar applications of the defense elicitors actigard ( acibenzolar - s - methyl ) , employ ( harpin protein ) , or odc ( chitosan ) to potted pear trees ( pyrus communis l. ) each caused an increase in mortality of cacopsylla pyricola ( frster ) ( hemiptera : psyllidae ) nymphs and altered the settling and oviposition behavior of the adults . in this study , we monitored c. pyricola populations over a 3-yr period on orchard - grown trees treated with water ( untreated control ) , actigard , employ , or odc . 
 fewer nymphs were observed on trees treated with elicitors compared with untreated trees in both 2014 and 2016 . a similar but statistically nonsignificant pattern was observed in 2015 when nearly 30% fewer nymphs were observed on trees treated with elicitors versus untreated controls . observed reductions in psyllid numbers by defense elicitors were modest and 
 do not warrant the use of these products alone for managing c. pyricola . 
 however , these products are often used for management of fire blight , and our observations that elicitors also reduce c. pyricola populations may be useful for system - wide integrated pest management approaches .