Identification of Host Fruit Volatiles from Three Mayhaw Species (Crataegus Series Aestivales) Attractive to Mayhaw-Origin Rhagoletis pomonella Flies in the Southern United States

Abstract

The apple maggot fly, Rhagoletis pomonella, infests several hawthorn species in the southern USA. Here, we tested the hypothesis that these populations could serve as reservoirs for fruit odor discrimination behaviors facilitating sympatric host race formation and speciation, specifically the recent shift from downy hawthorn (Crataegus mollis) to domestic apple (Malus domestica) in the northern USA. Coupled gas chromatography and electroantennographic detection (GC-EAD), gas chromatography with mass spectrometry (GC-MS), and flight tunnel bioassays were used to identify the behaviorally active natal fruit volatile blends for three of the five major southern hawthorns: C. opaca (western mayhaw), C. aestivalis (eastern mayhaw), and C. rufula (a possible hybrid between C. opaca and C. aestivalis). A 6-component blend was developed for C. opaca (3-methylbutan-1-ol [44%], pentyl acetate [6%], butyl butanoate [6%], propyl hexanoate [6%], butyl hexanoate [26%], and hexyl butanoate [12%]); an 8-component blend for C. aestivalis (3-methylbutan-1-ol [2%], butyl acetate [47%], pentyl acetate [2%], butyl butanoate [12%], propyl hexanoate [1%], butyl hexanoate [25%], hexyl butanoate [9%], and pentyl hexanoate [2%]); and a 9-component blend for C. rufula (3-methylbutan-1-ol [1%], butyl acetate [57%], 3-methylbutyl acetate [3%], butyl butanoate [5%], propyl hexanoate [1%], hexyl propionate [1%], butyl hexanoate [23%], hexyl butanoate [6%], and pentyl hexanoate [3%]). Crataegus aestivalis and C. opaca-origin flies showed significantly higher levels of upwind directed flight to their natal blend in flight tunnel assays compared to the non-natal blend and previously developed apple, northern downy hawthorn, and flowering dogwood blends. Eastern and western mayhaw flies also were tested to the C. rufula blend, with eastern flies displaying higher levels of upwind flight compared with the western flies, likely due to the presence of butyl acetate in the C. aestivalis and C. rufula blends, an agonist compound for eastern mayhaw-origin flies, but a behavioral antagonist for western flies. The results discount the possibility that the apple fly was "pre-assembled" and originated via a recent introduction of southern mayhaw flies predisposed to accepting apple. Instead, the findings are consistent with the possibility of southern mayhaw-infesting fly host races. However, mayhaw fruits do emit several volatiles found in apple. It is, therefore, possible that the ability of the fly to evolve a preference for apple volatiles, although not the entire blend, stemmed, in part, from standing variation related to the presence of these compounds in southern mayhaw fruit.

Identification of Fruit Volatiles from Green Hawthorn (Crataegus Viridis) and Blueberry Hawthorn (Crataegus Brachyacantha) Host Plants Attractive to Different Phenotypes of Rhagoletis Pomonella Flies in the Southern United States

“…A third described species, C. rufula, occurs primarily at the intersection of the eastern and western species' ranges between the lower Pearl River and Pascagoula River drainages in Mississippi and may represent a hybrid species; C. rufula does not appear to be infested by R. pomonella (Cha et al, 2011b, this issue). Our results are consistent with the possibility of endemic southern hawthorn-infesting host races of R. pomonella and do not support a "pre-adapted" apple race of the fly being present in the South (Cha et al, 2011b).…”

“…As done previously in Cha et al (2011b), differences in the frequency of upwind flight in the tunnel were compared using Fisher's Exact Test in R (R development core team, Vienna, Austria).…”

Geographic variation in fruit volatiles emitted by the hawthorn Crataegus mollis and its consequences for host race formation in the apple maggot fly, Rhagoletis pomonella

“…Full details of the methods used for headspace whole‐fruit volatile extract collection by ORBO32 activated charcoal filter traps (Supelco, Bellefonte, PA, USA), for gas chromatography‐mass spectrometry (GC–MS), and for coupled GC‐EAD are given in Cha et al. (,b). In brief, adsorbent collections for each sample of fruit were made over a 5‐day period at room temperature using the volatile trap.…”

“…The response of flies to host fruit volatiles was measured in a sustained‐flight tunnel using procedures as in previous studies with R. pomonella flies (Nojima et al., ; Linn et al., ,b; Cha et al., ,b). Results from previous R. pomonella flight tunnel studies using these procedures have been congruent with results from mark‐recapture studies on R. pomonella odour response behaviour in the field (Linn et al., , ; Forbes et al., ).…”

“…Adaptation to the diverse suite of southern Crataegus hosts infested by R. pomonella (Berlocher & Enquist, ) may have played a role in the subsequent evolution of the AP race in the northeastern USA. Some of these alternative Crataegus hosts have been shown to differ in their fruit volatile profiles, and flies infesting these different southern hawthorn species have been shown to preferentially behaviourally orient to their respective native host fruit volatiles (Cha et al., ,b). In addition, these southern hawthorn species – Crataegus aestivales (Walter), Crataegus opaca (Hook & Arn.…”