Monarch Butterflies Use Regenerating Milkweeds for ... · monarch adults that are produced late in the flight season of the butterfly. Additional key words: extension of breeding

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Monarch Butterflies Use Regenerating Milkweeds for Reproduction in MowedHayfields in Northern VirginiaAuthor(s): John Alcock Lincoln P. Brower Ernest H. Williams Jr.Source: Journal of the Lepidopterists’ Society, 70(3):177-181.Published By: The Lepidopterists’ SocietyDOI: http://dx.doi.org/10.18473/107.070.0302URL: http://www.bioone.org/doi/full/10.18473/107.070.0302

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VOLUME 70, NUMBER 3 177

Journal of the Lepidopterists’ Society70(3), 2016, 177–181

MONARCH BUTTERFLIES USE REGENERATING MILKWEEDS FORREPRODUCTION IN MOWED HAYFIELDS IN NORTHERN VIRGINIA

JOHN ALCOCK

School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, email: [email protected]

LINCOLN P. BROWER

Department of Biology, Sweet Briar College, Sweet Briar, VA 24595

AND

ERNEST H. WILLIAMS JR.Department of Biology, Hamilton College, Clinton, NY 13323

ABSTRACT. The effects of mowing milkweeds in areas visited by monarch butterflies (Danaus plexippus L., Nymphalidae) werestudied by counting the eggs and larvae on regenerating common milkweeds (Asclepias syriaca L., Apocyanaceae) in five adjacentmowed hayfields in northern Virginia in late summer 2015. At the same time monarch larvae were counted on mature senescentcommon milkweeds in unmowed areas adjacent or near to the mowed hayfields. Milkweeds supported populations of immaturemonarchs in both habitat types with initially many eggs and early instars found on regenerating plants in the mowed hayfields whilelate instars dominated the unmowed older milkweeds. As September proceeded, the censuses revealed an increase in the numbersof late instars on the mowed regenerating milkweeds whereas the abundance of larvae declined sharply on the older senescing milk-weeds, many of which had lost all or most of their leaves. The study showed that late season mowing of hayfields provided adult fe-male monarch butterflies with rejuvenated resources for reproduction during a time when senescent milkweeds were becoming un-suitable for the monarch larvae. Our findings have implications for managing land in ways to benefit monarchs and for mitigatingthe widespread decline of milkweeds, although the research raises several caveats and more needs to done to measure the fitness ofmonarch adults that are produced late in the flight season of the butterfly.

Additional key words: extension of breeding season by mowing, milkweed regeneration, monarch butterfly conservation

Monarch butterflies (Danaus plexippus) haveexperienced a dramatic population decline over the pasttwo decades and various factors have been proposed ascauses of this decline. Among the possible factors is (1)degradation of the Mexican high elevation forests ofOyamel fir (Abies religiosa, H.B.K., Pinaceae) wherethe monarchs overwinter (Brower et al. 2012, Vidal &Rendon-Salinas 2014, Brower et al. 2016) despite somesuccess in protecting and reforesting criticaloverwintering habitat (Vidal et al. 2014). Another majorcontributor to monarch declines is (2) the widespreadapplication of herbicides to herbicide-tolerantagricultural plants in the United States with theconsequent loss of farm field milkweeds (Pleasants &Oberhauser 2012). Moreover, monarch butterflies arealso affected by (3) the loss of extensive milkweedhabitats in the United States to housing developments,industrial expansion, and the reduction of the acreage inthe USDA Conservation Reserve Program (Taylor2014). Flockhart et al. (2015) considered the losses ofmilkweed breeding habitat in the United States to bethe key to understanding the population collapse of themonarch. As Taylor emphasized, government entities,conservationists, and the general public in the United

States should try to restore milkweed populations sothat the phenomenon of monarch butterfly migrationwill persist.Increased awareness of monarch decline, highlighted

by submission of a petition to the US Fish and WildlifeService to designate the monarch as a threatenedspecies (Crouch et al. 2015), has contributed to a focuson the possible causes of and suggested mitigations ofthe species’ collapse. For example, Freese & Crouch(2015) and Mirocha (2015) have documented themassive increase over the past two decades in acreageplanted with “Roundup Ready” corn and soybean cropsthat are genetically modified to resist the herbicideglyphosate that kills milkweeds and nectar sourceplants. The loss of milkweed has led severalorganizations (e.g., Journey North, Monarch JointVenture, Monarch Watch, The Xerces Society) toencourage the planting of milkweeds in home yards andgardens, along right-of-ways, as well as to question thefrequent mowing of roadside verges that often supportmilkweed populations. In addition, Fischer et al. (2015) and Baum &

Mueller (2015) have shown that appropriately timedmowing during the growing season can lead to the

regeneration of milkweeds, which provides a freshsupply of food for larval monarchs later in the seasonwhen most of the naturally growing milkweeds havesenesced. It is this last possibility that we explore innorthern Virginia.

MATERIALS AND METHODS

The abundance of monarch butterfly eggs andcaterpillars were monitored by the first author in fivelarge mowed hayfields at Monterey Farm, a 200+ acrefarm in Fauquier County, VA, near 38˚52'26"N,77˚54'21"W at approximately 165 m elevation. Most ofthe acreage of the farm is hayfield, and haying of thefields took place from 8 to 16 August 2015. The farmowners have an arrangement with a hay cutter thatspecifies a single cutting per summer and the retentionof unmowed buffer strips between wooded areas andthe hayfields.

On 30-31 August, regenerating common milkweeds,Asclepias syriaca L., in mowed areas were searched formonarch eggs and larvae, and twelve patches ofunmowed mature milkweeds were also examined.Counts were made every five days from 30 August to 23September, giving a total of six censuses, each requiring90–120 min. The unmowed patches in the buffer stripswere close to or adjacent to the mowed areas that wereinspected and so could be readily monitored. Noattempt was made to check the same plants in thehayfields on successive censuses. The main purpose ofthe study was to document whether the regeneratingmilkweeds attracted reproductive females by observingovipositing females and larvae in this habitat. Thehayfield milkweeds were largely A. syriaca but alsopresent was the significantly less common honeyvinemilkweed (Cynanchum leave (Michx.) Pers.) as well asvery uncommon (in the hayfields) butterfly weed (A.tuberosa L.). In addition, the censuses were designed toreveal if and when the unmowed common milkweedsnear the hayfields were simultaneously utilized bymonarchs. Observations made as the season progressedwere done to establish whether any caterpillars in thehayfields developed into mature larvae when the larvaewere no longer present on the mature milkweeds in thebuffer zones. This would demonstrate that thereproductive season of the butterfly had been extendedby cutting the hayfield grasses and weeds.

RESULTS

Common milkweeds had begun regenerating in largenumbers in the mowed hayfields by 24 August, 16 daysafter haying had begun and just a few days after rains on20 and 21 August had drenched the fields. By 1September, the milkweeds had grown substantially so

that many hundreds were already between 25 cm and50 cm in height, much taller than the grasses, whichwere slower to regrow (Fig. 1).

Between 3 and 16 September, observations of themowed hayfields yielded 15 records of ovipositingfemales, three of which were laying on the leaves of C.leave, demonstrating that both regenerating milkweeds

178178 JOURNAL OF THE LEPIDOPTERISTS’ SOCIETY

FIG. 1. (a) Young regenerating milkweeds, Asclepias syriaca,in the mowed hayfields (photographed on 24 August 2015). (b)Plants of A. syriaca that have regrown rapidly (photographed on1 September 2015). (c) A monarch butterfly egg laid on a leaf ofthe honeyvine milkweed (Cynanchum laeve) (photographed on3 September 2015). Photos: J. Alcock.

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did attract adult monarch females. On 17 Septembertwo females perched on larval foodplants and bent theirabdomens into the egg-laying position but did notoviposit on the selected plants. On 18 September andafterwards, no females were seen exhibiting ovipositionbehavior in the mowed areas; however, one egg wasfound in the mowed hayfields on 23 September.

Fig. 2 presents the results of the six censuses of themowed and the adjacent patches of unmowedmilkweeds. Although the milkweeds in the unmowedpatches were senescent in early September, theiryellowed, tattered leaves were nevertheless being eatenby substantial numbers of late instar caterpillars at theoutset of the study (Fig. 3). By the later censuses theunmowed milkweeds were occupied by only a fewlarvae of any size. In contrast, the regeneratingmilkweeds in the mowed field soon became populatedby early instar larvae, so that late instar caterpillars werepresent on the regenerating milkweeds through 23September. The significant comparisons in these dataare between the numbers of juvenile stages in the twohabitat types on the same date.

DISCUSSION

The consequences of using mowing to promoteregeneration by the common milkweed were examinedby Fischer et al. (2015) in upstate New York during anexperiment in which plots of milkweed-rich fields weremowed at three different times while control strips wereleft unmowed. The experimental and control strips werethen monitored for milkweed regeneration and theiruse by monarch butterflies. The authors reported thatmilkweeds in the mowed strips did indeed regenerateand attract adult female monarchs but only if themowing was done in July, not in August, to give time forthe plants to regrow at this more northerly latitude.Therefore, the timing of mowing was critical tomonarch reproduction. As the authors noted, theeffectiveness of mowing as a conservation measurealmost certainly depends not only on when the mowingoccurs but on local phenology at that geographiclocation and the species of milkweeds involved.

The conclusion of Fischer and co-authors issupported by the current study in northern Virginia inwhich mowing of hayfields was not completed untilmid-August and yet the milkweeds did regeneratesufficiently at this lower latitude to be used byovipositing female monarchs well into September. Theyoung regenerating milkweeds were quickly discoveredin mid-August by the butterflies, and relatively largenumbers of eggs and early instar larvae were quicklyproduced. In addition, females also had access to largenumbers of the honeyvine, another perennial milkweed

that regenerated strongly in the mowed fields. Incontrast, the unmowed areas provided acceptable foodfor monarch larvae only early in the study even thoughthe patches of A. syriaca at this time (mid-August) weredominated by senescent plants with yellowed anddamaged leaves. By mid- to late September thesesenescent milkweeds often were leafless, and as a result,few caterpillars were found on them. At this time,surviving caterpillars in the mowed hayfields wereprimarily large, late-instar larvae. These results suggestthat monarch reproduction in northern Virginia maybenefit from mowing portions of overgrown fields withcommon milkweeds fairly late in the season while alsomaintaining the senescent common milkweeds in theborders to these fields or in places nearby. In theunmowed patches, mature monarch larvae were presentin substantial numbers through 3 September whereasthose in the mowed field were present through 23September; that is, the breeding period was extendedby nearly three weeks. Mowing in New York, about 3˚latitude farther north than this Virginia site, extendedthe breeding period for at least two weeks, a similarlength of time. A primary difference between the resultsin the NY and VA sites was that mowing in NY on 17August produced very little milkweed regrowth,whereas mowing on the same date in the moresoutherly Virginia site did result in substantial new

FIG. 2. The number of eggs, small larvae (first, second andthird instars), and large larvae (fourth and fifth instars) found onsenescent Asclepias syriaca in unmowed borders to the hay-fields in comparison to the numbers of those found on regener-ating A. syriaca and honeyvine (Cynanchum laeve) in themowed hayfields. Six censuses were conducted from 30 Augustto 18 September 2015.

180180 JOURNAL OF THE LEPIDOPTERISTS’ SOCIETY

growth. In the future it would be valuable to conductmowings at similar times across several latitudinallydifferent sites to determine the degree to which thetiming of one or more cuts can extend the geographicbreeding windows. Much more work, however, remainsto be done to determine the best mowing regimes atdifferent latitudes.

Nevertheless, there are now two studies spanning 500km (approximately 3˚) of latitude indicating that mowingof late season milkweeds has the potential to providefresh plants for ovipositing females, resulting in anextended growing season for the larvae that feed uponthe regenerated milkweeds. If properly timed mowingwere instituted on a broad scale, the current widespreadloss of milkweeds suitable for monarch caterpillarscaused by the increasing use of herbicides on householdlandscapes, commercial agricultural crops, roadsides,and power and gas line right-of-ways might beameliorated to some degree. As seen in this study,

however, management of land for increased use bymonarchs may effectively combine regeneratingmilkweeds in mowed areas with uncut milkweeds innearby unmowed habitat, thereby providingcontinuously available larval hostplants during mid tolate summer extending the window of breeding.

There are, however, several caveats raised by thepresent study. The first is whether the loss of eggs andcaterpillars during the mowing of the fields iscompensated by reproduction on the regeneratingmilkweeds. As we have shown, the breeding season isextended but we do not know whether the offspring ofmonarchs produced during the lengthenedreproductive can migrate successfully to Mexico. AsBatalden and Oberhauser (2015) have demonstrated,senescing milkweed leaves are one of a constellation ofmigratory stimuli and their replacement by regeneratingyoung milkweeds could slow adults from enteringreproductive diapause and migrating. Another caveat is

FIG. 3. An unmowed patch of the common milkweed (Asclepias syriaca) photographed on 23 August 2015 at a time when thesenescent plants still possessed many yellow, damaged leaves most of which had fallen off by mid-September. (INSET) A fifthinstar monarch caterpillar that was feeding on the damaged leaves of an old milkweed growing in an unmowed patch photographedin September 2015. Photos: J. Alcock.

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that an extended breeding season could increase thepossibility of infection by the protozoan parasiteOphryocystis elektroscirrha (Bartel et al. 2011,Satterfield et al. 2015). The use of regeneratingmilkweeds could in other words be an ecological trapfor migrant adults that are induced by the availability offresh milkweed plants to reproduce at a time when theynormally would have little or no chance of increasingtheir production of viable young. Resolving these pointsis essential before we can say with certainty that the lateseason haying of fields with mature milkweeds providesa net benefit for monarch butterflies. However, thisstudy and that of Fischer et al. (2015) effectivelydocument that properly timed mowing can extend thetime available for monarchs to reproduce.

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Submitted for publication 30 October 2015; revised andaccepted 11 January 2016.