Host Discrimination by Adult and Larval European Apple Sawflies Hoplocampa testudinea (Klug) (Hymenoptera: Tenthredinidae) BERNARD D. ROITBERGI ANDRONALD J. PROKOPY Department of Entomology, University of Massachusetts, Amherst, Massachusetts01003 Environ. Entomol. 13: 1000-1003 (1984) ABSTRACT Adult and larval European apple sawflies (EAS), Hoplocampa testudtnea Downloaded from https://academic.oup.com/ee/article/13/4/1000/2393471 by guest on 29 September 2021 (Klug), were assayed for propensity to avoid resource sites (apple blossoms and growing apples, respectively) already occupied by conspecifics(i.e., assayed for host discrimination). Under laboratory conditions, adults oviposited significantly less frequently in blossomscon- taining oviposition wounds and eggs than in healthy, uninfested ones. Similarly, significantly fewer adults attempted oviposition in blossomscontaining artificial oviposition wounds than in intact blossoms.These results suggest that wound tissue exudates provide information to EAS adults on the infestation state of individual blossoms.EAS larvae, following emigration from their first hosts, invaded uninfested apples significantly more often than apples har- boring living conspecifics. Similar results were obtained when resident larvae were killed before testing. We were unable to determine the stimulus(i) larvae employ in the host- discrimination process. SEVERALORGANISMSliving upon restricted food Following oviposition, the female often feeds at resources are known to avoid oviposition sites al- the oviposition wound for variable periods of time ready occupied by conspecin.cs-i.e., they discrim- (Dicker 1954). Observations indicate that a female inate among hosts. Prokopy (1981a) discusses 26 does not usually lay more than one egg in a cluster species of phytophagous insects that produce and of blossoms (approximately five blossoms per clus- detect spacing or epideictic pheromones (sensu ter) before moving on to another cluster (Roitberg Corbet 1971) that deter oviposition by conspecifics and Prokopy 1980). at already occupied resource sites. In addition, some The newly hatched larva burrows under the fruit other phytophagous insects discriminate among skin, often encircling the entire fruit before mov- hosts by visually or acoustically assessing presence ing deeper toward the core. During this stage, the of conspecin.cs (e.g., Williams and Gilbert 1981). larva and the fruit are termed primary (Dicker The guild of insects which lives within and feeds 1954). The larva remains in the fruit until food upon the fruit of apple (Malus) does so under con- resources are exhausted. At this point, it emigrates ditions wherein both food and shelter are limited. and invades a second (secondary) fruit. Occasion- Previous investigations of two members of this ally, it may invade a third fruit if it exhausts re- guild in the eastern United States, the apple mag- sources of the second before completing develop- got fly, Rhagoletis pomonella (Walsh), and the ment. Larvae (and their new hosts) are termed codling moth, Cydia pomonella L., have demon- secondary once emigration from the primary host strated the existence and absence, respectively, of has occurred. host discrimination ability (Prokopy 1972, 1981b, Because food resources are limited for both pri- Roitberg and Prokopy 1983). Here we report evi- mary and secondary larvae, we were interested to dence for host discrimination by both adults and learn if EAS females avoid laying eggs in blossoms larvae of another member of the insect guild par- that already contain other EAS eggs, and if pri- asitizing apples (sensu Price 1977), the European mary larvae avoid entry into apples already oc- apple sawfly, Hoplocampa testudinea (Klug). cupied by secondary larvae. European apple sawfly (EAS) adults emerge in spring in the eastern United States when the first Materials and Methods apple blossoms open (Dicker 1954). Both sexes visit apple trees, where feeding, mating, and oviposi- Tests of Adults. In May 1980 and 1981 we col- tion take place. After feeding on nectar and pol- lected adult EAS from unsprayed blossoming ap- len, the female saws with her ovipositor into the ple trees at Hamden, Conn., and Amherst, Mass. receptacle of the blossom and inserts a single egg. Males and females were housed together in pie xi- glass screen cages (25 by 25 by 25 em) in the lab- oratory under fluorescent light at a photoperiod of I Present address: Center for Pest Management, Dept. of Bio- logical Sciences, Simon Fraser Univ., Burnaby, B.C. Canada. V5A LD16:8 between 50 and 60% RH, and 24°C. We 156 supplied them with water, honey, and fresh apple 1000 August 1984 ROITBERG AND PROKOPY: HOST DISCRIMINATION BY SAWFLIES 1001 blossoms as food and as sites for gaining oviposi- We induced dispersal of primary larvae by slic- tion experience (van Lenteren and Bakker 1975, ing into the apple with a scalpel. After 1 to 6 h, Roitberg and Prokopy 1981). the apple dried and the larvae left the fruit. We One day after collecting the sawflies, we tested then immediately removed the primary apple, their ability to discriminate between egg-infested leaving the larva for 4 h to choose either the con- and uninfested apple blossoms through the follow- trol or the treated apple during experiments 2 and ing procedure. We presented each female with a 3. For experiment 1, we did not remove the pri- fresh uninfested blossom attached to the end of a mary apple. In experiments 1 and 2, each second- probe (the blossom was kept turgid by placing a ary apple was dissected at the conclusion to deter- wet l-cm piece of Absorbal®around the cut stem). mine if it contained a secondary larva. We If the female oviposited, we removed her from the discarded data from any trial in which this was cage and placed her alone in a plexiglass screen not the case. Downloaded from https://academic.oup.com/ee/article/13/4/1000/2393471 by guest on 29 September 2021 cage (15 by 15 by 15 cm). Fifteen min later, we In experiment 3, all frass as well as the apple presented the female sawfly with either (a) one flesh immediately adjacent to the frass was re- uninfested blossom or (b) a blossom in which a moved from 10 infested secondary apples, blended different sawfly had previously oviposited ca. 1 h with 0.5 ml H20, and centrifuged at 10,000 rpm. earlier. If the sawfly accepted the blossom (i.e., In addition, an equal amount of apple flesh was oviposited in it), we allowed it to rest and repeated removed from 10 uninfested secondary apples, the procedure 15 min later. If the sawfly rejected blended with 0.5 ml H20, and t.:entrifuged. Apples the blossom (i.e., did not attempt to oviposit), we to be treated were cored with a no. 3 cork borer presented it with a second uninfested blossom. If (0.8 cm in diam). A l-cm wick of 0.6-cm Absor- the sawfly rejected that blossom as well, the data bal was inserted into the core hole. The moment for that trial were discarded. We continued to test the primary larva dispersed from the primary ap- sawflies in this manner from 5 to 11 h after the ple, we saturated the Absorbal with all (i.e., 0.5 lights were turned on. ml) of the supernatant. One apple received frass In addition to these experiments, we also tested extract, the other clean apple extract. for the propensity of females to discriminate be- tween uninfested, wounded blossoms (i.e., artifi- Results cially produced oviposition wounds) and uninfest- ed blossoms. To create wounds similar to those Tests of Adults. When we presented females produced by ovipositing sawflies we pierced blos- with apple blossoms in laboratory cages, they soms, at the calyx cup region, with a no. 3 insect readily crawled onto the blossoms and fed upon mounting pin. In most cases nectar exuded from nectar and pollen. The presence of a sawfly egg in the wound, as when sawflies oviposit. We followed some blossoms had no observable effect on the ten- the same protocol as in the aforementioned exper- dency to arrive or feed. However, the presence of iments. an egg did affect subsequent behavior. EAS fe- At the end of each test, we dissected all blossoms males spent significantly less time on the recepta- to determine the presence or absence of eggs. cle of an uninfested blossom before either attempt- Tests of Larvae. We collected sawfly-infested ing oviposition or leaving than did females on an apples from unsprayed trees at Hamden, Conn., egg-infested blossom (2.5 sec ± 0.4 SE, n = 35 ver- Amherst, Mass., and Durham, N.H. The apples sus 5.2 sec ± 0.8 SE, n = 33, P ~ 0.003, t test). were stored at 4°C until required. We tested the There was no statistically significant (P > 0.4, t response of primary larvae to: uninfested second- test) difference in search time between females ary apples (controls); apples infested by secondary which oviposited in infested blossoms versus those larvae, as evidenced by copious fresh frass at the that rejected these. entrance hole; apples infested by secondary larvae A significantly smaller percentage of arriving freshly killed by freezing for 8 h to eliminate pos- females attempted oviposition in blossoms that sible acoustical cues (control apples in this test were contained eggs than in uninfested ones (48%, n = also frozen); and uninfested secondary apples 79 versus 83%, n = 63; P < 0.001, G test [Sokal treated with an extract of secondary larval frass. and Rohlf 1969]). Similarly, significantly fewer fe- All secondary apples were of equal size (ca. 2.5 males attempted oviposition in blossoms with ar- cm in diam). tificial oviposition wounds than in untreated blos- All tests were conducted in 240-ml Dixie cups. soms (33%, n = 21 versus 83%, n = 63; P < 0.001, In each test, we placed one primary infested apple G test).