Rosenheim, Jay A.

Wilhoit, Lawrence R. : L. R. Wilhoit was Postgraduate Researcher (currently an Environmental Research Scientist for the Department of Pesticide Regulation, California EPA), Department of Entomology, UC Davis

yield losses were apparent . As has been Why lacewings may fail to suppress aphids . . . found in studies of several different cot - ton insect pests , plants that are setting bolls appear to have limited abilities to compensate for feeding damage . During Predators that eat other the late season , when bolls are opening and cotton lint is exposed , cotton aphids create problems by excreting large quan - predators disrupt cotton tities of sugary honeydew , which fall onto lint and create â?? sticky cotton . â?쳌 Problems with sticky cotton become ap - aphid control parent during harvest , ginning and yarn manufacturing , and threaten overseas markets and the price premiums Califor - nia cotton has historically received . Be - Jay A . Rosenheim D Lawrence R . Wilhoit cause the cotton aphid is already resis - tant to many insecticides in California and an even larger array of pesticides in predators may attack other predators , the southern United States , long - term Thepredatory green lacewing , with potentially negative effects on pest management of aphids will probably Chrysoperla carnea , is often control . Here , we report a study de - need to rely on noninsecticidal alterna - abundant in mid - and late - season signed to determine the effectiveness of tives . lacewing larvae , Chrysoperla carnea , as cotton fields in the San Joaquin Cotton grown in the San Joaquin Val - biological control agents of the cotton ley generally develops large populations Valley . However , neither these aphid , Aphis gossypii , which feed on of generalist predators , including big - natural populations nor insecfary - mid - and late - season cotton in the San eyed bugs ( Geocoris spp . ) , damsel bugs reared and mass - released lace - Joaquin Valley . We found that a number ( Nabis spp . ) , assassin bugs ( Zelus spp . wings appear to suppress popula - of generalist predators impose heavy ( Orius and others ) , minute pirate bugs mortality on lacewing populations and of the cotton aphid . The key tions tristicolor ) and lacewings ( primarily thereby render natural and augmented Chrysoperla carnea ) . Other predators , less reason for the ineffectiveness of populations of lacewings ineffective as abundant , are also present . Lacewing biological control appears to be biological control agents . larvae , known as potential predators of the heavy mortality imposed on The cotton aphid , a major pest of cot - aphids in many crops , are available from lacewing larvae by other general - ton in California , presents very different several commercial insectaries . Some problems for cotton production at differ - ist insect predators . Results of a growers use augmentative releases of ent times of the year . Early season popu - purchased lacewings at the recom - study suggest that interactions lations , which develop on very small mended rate of 5,000 eggs per acre to between different species of in - cotton seedlings ( often on plants with improve control of cotton aphids . sect predators may disrupt the fewer than six nodes ) , can cause crinkled Natural densities of lacewings of pest species . biological control leaves , partial defoliation and stunted plant growth . Nevertheless , research still The first question addressed was : in progress suggests that this damage is How many lacewing eggs and larvae are fully compensated for before harvest naturally present in late - season cotton Rising costs of insecticides , widespread and that the timing of crop maturation , fields when sticky cotton is a potential insecticide resistance and increasing re - quantity of yield and cotton fiber quality problem ? Six fields were sampled be - strictions on insecticide use in California are unaffected . At mid - season , cotton tween August 26 and September 2,1992 . have spurred interest in insect manage - aphid populations are frequently low ; In each field , entire plants were sampled ment by other means , including biologi - however , during 1992 , populations in and carefully inspected to count all cal control . Generalist insect predators the southern San Joaquin Valley grew lacewings present . The number of plants are frequently abundant in annual crops , rapidly during July and August , and per row meter was recorded to translate including field and vegetable crops , and counts into lacewings per acre . As have been identified as important in shown in figure 1 , natural densities of suppressing populations of damaging lacewing eggs were in all cases high , insects . However , effective use of such ranging from 77,000 to 380,000 per acre . natural enemies to manage pests re - Clearly , these naturally present eggs quires more complete understanding of were so abundant in the sampled fields insect ecology , including the biology of that more releases of insectary - reared insect predators . lacewing eggs at the recommended rate In cotton , for instance , generalist ( 5,000 per acre ) would not be useful . Ob - predators may be critical in controlling servations of many other fields suggest populations of spider mites , Lygus bugs that lacewings establish large densities and several worm pests . Few pest man - Y - 5 of eggs in cotton fields at midseason and agement experts or insect ecologists 2 that these densities remain fairly stable have studied interactions between insect 5 for nearly all of the remainder of the predators , leaving the impression that P 7 growing season . Thus , before releasing insect predators feed exclusively on her - lacewings , sampling of naturally present Adults and nymphs of the cotton aphid , bivorous arthropods . However , it is pos - lacewings is recommended to determine Aphis gossypii . sible in theory that some generalist CALIFORNIA AGRICULTURE , SEPTEMBER - OCTOBER 1993 7 food limitation , was causing heavy mor - bugs , prey on lacewing larvae . Other predators that were abundant , for tality of young lacewing larvae . example damsel bugs , were never ob - Lacewing egg releases served feeding in the field on any prey ; thus , we were unable to determine if Methods . Experiments to assess the they preyed on lacewing larvae . To de - effect of lacewing egg releases were con - termine if generalist predators caused ducted at the Kearney Agricultural Cen - substantial mortality of lacewing larvae , ter and the UC Cotton Research Station and to quantify their effect on the bio - at Shafter in 1991 . Two treatments , the logical control of the cotton aphid , we release of 5,000 insectary - reared lace - performed a manipulative experiment . wing eggs and no eggs released , were Methods.This experiment , con - replicated ten times in a randomized ducted August 21 - 30 , 1992 , at the block design . Our release rate , about 260 Kearney Agricultural Center , was de - times the recommended rate of 5,000 signed to isolate the influences of each of eggs per acre , was chosen to determine three predators , big - eyed bugs , damsel whether any effect from lacewings could bugs and assassin bugs , with and with - be discerned ; smaller releases would out the presence of lacewings , on lace - have been dwarfed by naturally present wing survival and biological control of populations . Releases at these rates are not , however , commercially feasible . the cotton aphid . Individual plants har - Each plot was 10feet by 6 rows of cot - boring aphid populations were chosen ton , cultivar GC - 510 . Two days before for study . Each plant was carefully in - the lacewings were released and weekly spected to count all aphids and to re - thereafter , aphid densities in each plot move all nymphal and adult predators . were quantified by sampling ten leaves Polyester mesh sleeves were then per plot and counting all aphids . To de - placed over the plants to create a small termine the impact of the lacewings , we cage ( either the entire plant or the top portion of the plant was enclosed , de - used the ratio of postrelease aphid counts to prerelease aphid counts . pending on aphid density ) . Before seal - Results.Because the effects of the ing the sleeve cage , eight treatments , lacewing releases were similar in the each replicated 6 to 10 times , were then two experiments , the results are com - applied to the caged plants : ( 1 ) no pred - ators , an â?쳌 aphids only â?쳌 treatment ; bined in figure 2 . For 3 weeks following ( 2 ) the lacewing releases , aphid densities aphids plus two big - eyed bug adults ; remained approximately constant in the ( 3 ) aphids plus two damsel bug adults ; release plots but expanded in the non - ( 4 ) aphids plus two assassin bug adults ; ( 5 ) aphids plus five C . carnea lacewing release , control plots . By the second week , the aphid population in the re - lease plots was less than before release ( ratio = 0.85k 0.16 ) , while the aphid population had more than doubled in the control plots ( ratio = 2.37 & 0.67 ; t = 2.50 , P = 0.03 ) . This trend continued for a third week , but for the remaining period there was little or no difference between the two treatments ( fig . 2 ) . Thus , releases of extremely high densities of lacewing eggs produced only a modest and tran - sient suppression of aphid populations . We suspect that releases conducted at the recommended rate would not have produced detectable results . Fig . 1 . Naturally occurring densities of A few days after the lacewing eggs lacewing eggs and larvae in six fields were released , few lacewing larvae sampled during the late season ( August could be found in any of the plots . This - September 2,1992 ) in San Joaquin 26 Adult assassin bug , Zelus renardii . Assas - observation was consistent with many Valley cotton . Five plants were sampled in sin bugs were found to be major predators other observations where many adult each field except for field S1 , in which 48 of lacewing larvae . plants were sampled . Field K had been lacewings and their eggs were seen , but treated with methamidophos June 24 for few or no larvae were detected ( fig . 1 ) . Lygus control , and field S3 had been whether releases will substantially in - However , a number of other generalist treated with chlorpyrifos August 4 for crease densities in the field . predators were present . aphid control ; all other fields were never Although egg densities were uni - treated . Shown are means plus one stan - Predators attack lacewings formly high , in four of the six fields we dard error of the mean . No lacewing larvae recovered no lacewing larvae ( fig . I ) , de - Direct observations in the field in were recovered from fields WS1 , K , S2 spite the fact that in several of the fields 1991and 1992revealed that generalist S3 . WS = West Side Field Station ; K or cotton aphid prey were abundant . This predators , including nymphal big - eyed = Kearney Agricultural Center ; S = UC suggested that some factor , other than Cotton Research Station , Shafter . bugs and nymphal and adult assassin 8 CALIFORNIA AGRICULTURE , VOLUME 47 , NUMBER 5 when other predators were added . AI - logical control researchers . Densities of larvae ( second instar ) ; ( 6 ) aphids plus lacewing eggs are naturally high in San though big - eyed bugs had a minimal ef - five lacewing larvae plus two big - eyed Joaquin Valley cotton fields ; thus , aug - fect ( P = 0.15 ) , the addition of damsel bug adults ; ( 7 ) aphids plus five lacewing mentative releases of lacewings should bugs allowed aphid populations to start larvae plus two damsel bug adults ; ( 8 ) only be contemplated after field scout - growing again ( P = 0.01 ) , and the addi - aphids plus five lacewing larvae plus ing has shown that natural populations tion of assassin bugs appeared to re - two assassin bug adults . Lacewing lar - are not present in high densities . Our move most of the suppressive effect of vae were obtained from a commercial in - small plot releases suggest that releases sectary . After 7 to the lacewings 8 days , the plant ( P = 0.001 ; fig . 3b ) . of lacewing eggs at extremely high rates stems were cut and the sleeve cages Clearly , these predators did not act in an ( much higher than is commercially vi - brought into the laboratory , where all additive way to suppress aphid num - able given the cost of predators and aphids present were bers . Rather , the heavy predation by $ 3 to $ 4 per 1,000 eggs ) produced only modest suppres - counted . damsel bugs and assassin bugs on lacewing larvae allowed aphids to es - Results . Survival of lacewing larvae sion of aphids . Recommended release cape the effective biological control pro - varied strongly across the four treat - rates would probably not produce de - duced by the lacewings alone . This re - ments to which lacewings were added tectable results . The key reason why naturally present sult was surprising , especially for ( fig . 3a ) . Survival decreased from 47 % in the absence of predators to damsel bugs , which are known as major populations and augmentatively re - 20 % when predators of aphids . leased lacewing eggs do not control big - eyed bugs were present ( P = 0.06 ) , aphids appears to be the heavy mortality and to 0 % when either damsel bugs Conclusions imposed on lacewing larvae by other ( P = 0.001 ) or assassin bugs were generalist predators . Because our con - Biological control of the cotton aphid present ( P = 0.003 ) . We can infer that clusions are based on only the decreased lacewing survival was a 2 years of by generalist predators involves com - result of predation rather than competi - study , additional work is required to de - plex interactions among predators that termine how general our results are . We have not previously been studied by bio - tion for aphid prey , because the avail - ability of aphid prey was greater where need to understand fully the interactions between lacewings and other generalist predatory bugs were present . This nega - tive effect of the generalist predators on predators to predict late - season out - breaks of the cotton aphid . lacewing survival influenced the level of Can we manipulate this system to in - biological control of the cotton aphid crease the effectiveness of lacewings ? ( fig . 3b ) . The first five treatments shown We do not wish to suppress populations in figure 3b provide a test of whether of big - eyed bugs or damsel bugs because each of the four predators considered these predators probably help control alone has an impact on aphid population other cotton pests , including Lygus bugs , growth . When aphids were caged alone , their populations expanded rapidly , as spider mites and worms . Thus , we may need to search for other biological con - expected . Assassin bugs and big - eyed bugs alone exerted minimal influences trol agents for the cotton aphid that are on the rate of aphid population growth not susceptible to these predators , or de - velop other control tactics , such as host ( P > 0.10 ) . Damsel bugs slowed aphid plant resistance or cultural techniques . = 0.03 ) , population growth substantially ( P Research continues on all of these fronts . but did not control the populations . Lace - wing larvae were , however , able to cause aphid populations to decrease rather than expand P = 0.001 ) ; the me - 1 . A . Rosenheim is Assistant Professor and L . R . Wilhoit was Postgraduate Researcher dian response was a 91.2 % decrease of aphid numbers . ( currently an Environmental Research Scientist for the Department of Pesticide This highly successful level of bio - Regulation , California EPA ) , Department logical control produced by lacewing of Entomology , UC Davis . larvae feeding alone was disrupted The authors thank Christine Armer and Paul Wynholdsfor technical assistance ; Thomas Leighfor the use offield cages ; Kent Les Ehler and Thomas Leighfor Daane , helpful discussions ; and Sinthya Penn and William Lalorfor comments on this manu - script . They thank the staffof the Kearney Agricultural Center and the UC Cotton Re - search Station , Shafler , for growing the experi - mental cotton plantings , and the Rincon - Fig . 3 . Influences of generalist predators yfor providing lacewing eggs Vitoua lnsecta ( B ) per - on ( A ) lacewing larval survival and for testing . capita aphid population growth rates . Per - This study wasfunded by cotton - grower capita aphid population growth was calcu - funds made available through Cotton lncor - - initial lated as ( final aphid numbers porated , in cooperation with the California aphid numbers ) / ( initial aphid numbers ) ; a Fig . 2 . Field experiment assessing the State Support Committee , Rick Wegis , value of 0.0 indicates no change in aphid efficacy of augmentative releases of green chairman , and the University of California populations . Aphids were present in all lacewing eggs for biological control of the treatments . cotton aphid . Statewide lPM Project . CALIFORNIA AGRICULTURE , SEPTEMBER - OCTOBER 1993 9