The role of vernal pools in the 1992 mass dispersal of Vanessa cardui (Nymphalidae) with new larval hostplant records

3 Pages·1993·2.3 MB·English

by  Carol W Witham

JournalofResearchon theLepidoptera 30(3-4):302-304
Note
The Role ofVernal Pools in the 1992 Mass Dispersal ofVanessa
cardui (Nymphalidae) with New Larval Hostplant Records
Vernalpoolsaresmalldepressions,underlainbyhardpanordenseclay,inthe
grasslands ofthe Cental Valley. Winter rainfall perches above the impervious
subsoil and slowly evaporates as a result of increasing temperatures in the
spring. Duringmid-to-latespring,vernalpoolsareislandsoffreshvegetationin
an otherwise drylandscape. The floweringforbs within the pools are predomi-
nantly nativewith a high proportionofendemics. Manyoftheseplants require
aspecificinundationperiod. Thisresultsinthevernalpoolscontainingconcen-
tric rings ofdifferent plants, each with theirown uniquely coloredblooms. The
historic extent of vernal pools has been severely diminished by agricultural
conversionandurbanexpansion. Forallofthesereasons,vernalpoolshavelong
attracted botanists.
In western Tehama County, several areas ofvernal pool formations remain.
DuringavisittothesepoolsonApril14-16th,IobservedthemassVanessacardui
migrationthroughthearea. Thepaintedladieswereflyingingreatnumbersin
NNW
a direction with individuals often stopping to nectar on Navarretia
leucocephalaBentham(Polemoniaceae). Insomesmall(<30sq.meters)pools,as
many as 50 individuals were nectaring on this low growing, white-flowered
vernal pool endemic. The Vanessa cardui were notobserved to nectaron other
species even though several were blooming.
During a subsequent visit on April 27-29th, the numbers ofVanessa cardui
migratingthroughtheareaweresignificantlylower. Uponcloseinspectionofthe
vernal pool plants, I observed many early instar larval nests on Psilocarphus
brevissimus Nuttall (Asteraceae: Inuleae), another vernal pool endemic. Ran-
domtransectplacementsinseveralofthesmall, shallowvernalpoolsyeildedan
average of>100 larval nests per square meter. Larger pools, which had been
partiallyinundatedduringtheheightofthemigration,hadmanylarvalnestson
the margins and few in the center. Extremely deep pools, which had been
completely inundated duringthe migration had only an occasional larval nest.
Manythird and fourth instarlarvae were alsoobserved duringthe late-April
field visit. These larvae had left their nests and were apparently in search of
fresher food. Most were eating adjacent Psilocarphus brevissimus. However,
somelarvaewereobservedonPsilocarphustenellusNuttall,Micropuscalifornicus
Fischer & Meyer and Filago gallica Linnaeus (Asteraceae: Inuleae), plus
AchyrachaenamollisSchauer(Asteraceae:Madiinae)andPlagiobothrysstipitatus
var.micranthus(Piper)Johnston(Boraginaceae). AlthoughEryngium(Apiaceae)
is cited as a larval host plantin TheButterflies ofNorthAmerica (Scott, 1986),
none were observed on the Eryngium vaseyi Coulter & Rose occurring in the
vernalpools. SeverallargelarvaewerecollectedfromtheTehamaCountyvernal
pool site. These were successfully reared on Psilocarphus brevissimus with
pupation beginningon May 2nd and emergence beginningon May 9th.
A final field visit was conducted on May 5-6th and many newly emerged
Vanessa cardui were observed migrating. However, the preceeding week had
been 15-20° Fabove local averagetemperature andmostoftheremainingearly
30(3-4):302-304 303
Photo 1: Typical vernal pool from western Tehama County.
Photo 2: Psilocarphus brevissimus, a vernal pool endemic plant and new larval
hostplant record for Vanessa cardui.
304 J. Res. Lepid.
instarlarvaewerefounddeadintheirnestsonthedessicatedhostplant. Larger
larvae were observed movingfrom plant to plant.
Following the discovery of Vanessa cardui larvae on vernal pool endemic
plants,aquickreconnaissanceofvernalpoolsinGlenn,Colusa,Sacramentoand
Solano counties was made. On May 5-7th, several vernal pool formations were
visited and Vanessa cardui larvae orlarval nests were observed ateach site on
Psilocarphusbrevissimus. AttwositesinSolanoCounty,laterinstarlarvaewere
also observed onEvaxcaulescens (Bentham) Gray(Asteraceae: Inuleae).
Ofconsiderable significance is the lackofadultnectar sources at some ofthe
vernalpoolsites. TheterracesoilformationvernalpoolsinTehamaCountyare
dominated by Navarretia leucocephala and Psilocarphus brevissimus which
placesanadultnectarsourceandthehostplantincloseproximity. However,the
basin soil formation vernal pools in Solano County contain very little or no
NavarretialeucocephalaandrelativelysmallamountsofPsilocarphusbrevissimus.
Nevertheless,thesepoolscontainedasimilarratiooflarvaetohostplant. From
these observations we may deduce that use ofPsilocarphus brevissimus as a
larval hostplantwas not purely adventitious.
Carol W. Witham, 1028 CypressLane, Davis, California 95616.