The observed sex ratio ofPediobius foveolatus [Hym.: Eulophidae] in

ENrOMOPHAGA, 22 (2), 1977, 175-177
THE
O B S E R V E D SEX R A T I O OF P E D I O B I U S F O V E O L A T U S
[ H Y M . : E U L O P H I D A E ] I N F I E L D P O P U L A T I O N S OF
EPILACHNA VAMVESTIS
[COL. : C O C C I N E L L I D A E ] (1)
L. M. STEVENS, J. U. M c G u m E , A. L. STEINHAUER & P. A. ZUNGOLI
Department of Entomology, University of Maryland, College Park, Maryland 20742, U.S.A.
Larvae of Epilaehna varivestis MtJLSANT, parasitized by Pediobius foveolatus (CRAWFORD), were collected in soybean fields in Maryland during 1975.
Adult P. foveolatus emerging from the larvae were sexed and counted. The ratio
of males to females increased as the availability of food within the host decreased.
This information is useful in development of exponential models predicting hostparasite interactions.
Pediobius Joveolatus ( C R A W F O R D ) , a parasite of epilachnine larvae from India
(ANGALET et al., 1968), is presently used in the state of Maryland, U.S.A., as a biological control for suppressing Epilachna varivestis MULSANT populations on soybeans
(STEVENS et al., 1975a). Because the parasite does not overwinter, it must be released
annually. To predict expected increase in the parasite population with exponential
models, a reliable estimate of the sex ratio is required (McGuIRE, unpublished manuscript). The sex ratio of a sample of field collected P. foveolatus was 1 ~':1.33 ~, but
laboratory-reared parasites had a sex ratio of 1 ~:6.75 9 (STEVENS et al., 1975b).
These widely divergent ratios indicated that a dynamic situation existed. The following
study was initiated to clarify the seasonal changes in sex ratio under conditions in
Maryland.
MATERIALS AND METHODS
P. Jbveolatus adults were released throughout the M a r y l a n d soybean growing
area f r o m mid-June to mid-July, 1975. The parasite p o p u l a t i o n increased and dispersed
so that by September, parasitized larvae of E. varvivestis were present in almost all
soybean fields infested by the beetle. F r o m September 5 through October 4, 922 parasitized larvae were collected at r a n d o m .from various fields. These larvae were held
individually until the adult parasites emerged. N u m b e r s emerging f r o m each parasitized larva were recorded and the individual parasite adults were sexed, based on
presence or absence of the ovipositor. Sex ratios were calculated for each collection
date. Sex ratios were also calculated for adult parasites based on the numbers emerging
(1) Scientific Article No. A2208, Contribution No. 5189 of the Md. Agric. Exp. Sta. This study
was supported through U.S.D.A. Cooperative Agreement No. 12-14-1001-41 betweerl the U.S.D.A.A.R.S. Beneficial Insect Introduction Laboratory and the University of Maryland.
L.M. STEVENS et al.
176
f r o m each parasitized larva. For ease in analysis, the results were grouped in increments of 5 (i.e. 1 to 5, 6 to 10, l l to 15, etc. adult P.foveolatus emerging per parasitized
larva).
RESULTS AND DISCUSSION
There was no apparent relationship between sex ratios of parasite adults and
collection date. However, when the data were grouped on the basis of numbers of
adults emerging per parasitized larva, the sex ratio decreased as the number of adults
emerging per parasitized larva increased (table 1). The average sex ratio of 1 to
20 adults that emerge per larva is 1 c?:3.29 ~. I f more than 20 adults per larva emerge,
the sex ratio is almost halved immediately and continues to decline as numbers increase.
I n prior l a b o r a t o r y studies (STEVENS et al., 1975b) the average n u m b e r of parasite
adults emerging f r o m a larva parasitized by a single oviposition was 11.1 and ca. 95 ~o
of the time the range involved was 1 to 21 parasites. On this basis one might state
that the parasitized larvae in the 1st 4 groups in table 1 (with 1 to 20 parasites emerging)
represent those that received a single oviposition. Those with more than 20 emerging
adult parasites p r o b a b l y reflect multiple (2 or more) ovipositions, a situation that
occurs as a result of host scarcity in relation to ovipositing females. This state is
achieved in the field as host larvae decline in numbers or abundance of P. foveolatus
increases.
TABLE 1
The sex ratio of Pediobius foveolatus dependent on number of parasites
per mummy collected in Maryland soybean fields during the period of
September 5 to October 4, 1975
Range category
of the number
of parasites
emerging per mummy
Number
of mummies
contributing parasites
to sex ratio
Sex ratio
~' / ?
1-5
6-10
11-15
16-20
21-25
26-30
31-35
36-40
41-100
107
232
120
ill
96
76
68
41
71
1 : 3.23
t : 3.59
1 : 2.85
1 : 3.18
1 : 1.78
t : 1.28
1 : 0.91
1 : 0.71
I : 0.48
That the sex ratio of d' : ~ declines as the n u m b e r of parasites developing within
the host larva increases is evident. The reason for this association is less apparent.
P. foveolatus is a n arrhenotokous species (STEVENS et aL, 1975b). Sex of developing
individuals is determined prior to egg hatch within the host larva. Therefore it would
appear likely that environmental factors within the host larva could impose a differential determination and/or survival between the sexes (KERR, 1962). The obvious
factor is the reduced availability of food as numbers of developing parasites increase
within each host larva. Another possible factor is that ovipositing female parasites
SEX R A T I O OF
Pediobius foveolatus
177
are able to c o n t r o l f e r t i l i z a t i o n o f eggs as t h e y are l a i d in r e s p o n s e to a n e x t e r n a l stimulus,
such as the presence o n the h o s t o f a m a r k i n g p h e r o m o n e left d u r i n g p r i o r o v i p o s i t i o n s .
R e g a r d l e s s o f the m e c h a n i s m , it a p p e a r s t h a t the sex r a t i o declines m a r k e d l y as h o s t
a v a i l a b i l i t y declines. E v o l u t i o n o f this m e c h a n i s m w o u l d be a d v a n t a g e o u s to the
p a r a s i t e species in t e r m s o f c o n s t r a i n i n g the n u m b e r o f o v i p o s i t i n g f e m a l e s d u r i n g
h o s t scarcity.
V a r i o u s f a c t o r s affect the p o p u l a t i o n d y n a m i c s o f E. varivestis. O n e f a c t o r is
the c h a n g i n g sex r a t i o o f the p a r a s i t e , P. foveolatus, u n d e r different e n v i r o n m e n t a l
c o n d i t i o n s . Sex r a t i o i n f o r m a t i o n is i n d i s p e n s a b l e to d e v e l o p m e n t o f e x p o n e n t i a l
m o d e l s to f o l l o w t h e p r o b a b l e p o p u l a t i o n i n t e r a c t i o n s b e t w e e n h o s t a n d p a r a s i t e .
W e c o n c l u d e t h a t the sex r a t i o o f P. foveolatus c h a n g e s d u r i n g the s e a s o n a c c o r d i n g
to t h e r a t i o o f the p a r a s i t e to its host. A t a low r a t i o o f p a r a s i t e to host, t h e sex r a t i o
f a v o r s females, b u t at a high r a t i o o f p a r a s i t e to host, the sex r a t i o f a v o r s males. A n y
realistic m o d e l o f E. varivestis a n d P. foveolatus i n t e r a c t i o n m u s t a c c o u n t for this
o b s e r v e d c h a n g i n g sex r a t i o .
ACKNOWLEDGMENTS
We thank the following students at the University of Maryland : CAROL GOLDS'rE~, BEVERLY
JANE HEACOCK, STEVE LAURSEN, RHONDA ROLLINS, and INGRID M. SUNZENAUER.
RI~SUMt~
Rapport des sexes de Pediobius foveolatus [Hym. : Eulopkidae] dans les populations naturelles
de Epilaekna varivestis [Col. : Coccinellidae]
Des larves de Epilackna varivestis MULSANT, parasit~es par Pediobius foveolatus (CRAWEORD)
ont ~t6 r~colt~es en 1975 dans des champs de soja du Maryland. Les adultes de P. foveolatus issus de
ces larves ont ~t6 d6nombr~s et les sexes reconaaus. Le nombre de mfdes par rapport aux femelles augmente
lorsque la disponibilit~ de la nourriture offerte par l'h6te diminue. Ces renseignements sont utiles pour
l'6tablissement de modules exponentiels destin6s ft. pr~voir les interactions h6tes/parasites.
REFERENCES
ANGALET, G.W., COLES, L.W. & STEWART, J.A. - - 1968. Two potential parasites of the Mexican
bean beetle from India. - - J. Econ. Entomol., 61, 1073-1075.
KERR, W.E. - - 1962. Genetics of sex determination. - - Annu. Rev. Entomol., 7, 157-176.
STEVENS, L.M., STEINUAUER,A.L. & COULSON, J.R. - - 1975a. Suppression of the Mexican bean beetle
on soybeans with annual inoculative releases of Pediobius foveolatus. - - Environ. Entomol., 4,
9474952.
STEVENS, L.M., STEINHAUER,A.L. & ELDEN, T.C. - - 1975b. Laboratory rearing of the Mexican bean
beetle and the parasite, Pediobius foveolatus, with emphasis on parasite longevity and host parasite
ratios. - - Environ. Entomol., 4, 953-957.