Journal of Insect Science (2007) 7 (16), 3-4

Andrew Chow and Kevin M. Heinz (2007)
Use of host-size-dependent sex ratio theory in parasitoid mass rearing
Journal of Insect Science 7 (16), 3-4
XIV International Entomophagous Insects Workshop - Jun. 11-15, 2006, Newark, Delaware, U.S.A.
Abstract: An inherent problem with augmentative biological control is the high cost for the large numbers of control agents needed for suppressing pests. Overproduction of males in parasitic Hymenoptera contributes to higher costs for biological control because only females kill pests directly. We present a technique, based on manipulation of female parasitoid behavior and host composition, for generating less male-biased sex ratios in parasitoid species that adjust their sex allocation in response to relative host size. Our technique used predictions generated from host-size-dependent sex ratio theory to develop a system for decreasing the proportion of males produced in mass rearing of Diglyphus isaea (Walker) (Hymenoptera: Eulophidae), a commercially available parasitoid of agromyzid leafminers. We present findings on: (1) mechanisms used by female wasps for assessment of host size, (2) how host-size information is used to make sex allocation decisions, (3) how sex allocation behavior can be manipulated in a commercially feasible manner, and (4) differences in wasp quality when female production is maximized. First, we identified behaviors and measurements made by female wasps that correlate well with the probability that a given sex will be laid. We then asked whether naive or experienced wasps assessed host size differently. Two parameters were important for predicting sex allocation: wasp transits and time for host paralysis. Wasp experience affected the extent but not the types of parameters that predict sex allocation. Second, we used host quality and memory models to show how female wasps are influenced more by recent than past host encounters in their assessment of host size for sex allocation. Third, we compared the offspring sex ratios of female wasps presented with different compositions of hosts. Presenting females with increasingly larger hosts over 1-2 days reduced mean sex ratio; however, females produced more male-biased sex ratios (similar to commercial units) if exposed continuously to only large hosts. Increasing host size over time has limited application for insectaries. Thus, we tested whether sex allocation could be modified by the availability and distribution of two different-sized hosts. We then compared the use of both small hosts and large hosts to only large hosts for simulated mass rearing of wasps over 8 weeks. Using both small hosts and large hosts produced similar numbers of wasps as using only large hosts, but reduced mean sex ratio of weekly cohorts from 66% male to 56% male. The two techniques produced females of similar size, but using both small hosts and large hosts produced slightly smaller males than using only large hosts. Finally, we compared control of agromyzid leafminers with releases of identical numbers and sex ratios of wasps produced by the two rearing technique. In trials simulating infestation of greenhouse chrysanthemums during an 11-week crop cycle, we found no significant differences between the levels of control obtained with wasps produced by either rearing technique. Adoption of our rearing technique by commercial insectaries could reduce implementation costs for not only D. isaea but also other parasitoids that show host-size-dependent sex allocation.
(The abstract is excluded from the Creative Commons licence and has been copied with permission by the publisher.)
Database assignments for author(s): Andrew Chow, Kevin M. Heinz

Research topic(s) for pests/diseases/weeds:
biocontrol - natural enemies
Research topic(s) for beneficials or antagonists:
rearing/culturing/mass production

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
Diglyphus isaea (parasitoid)