Influence of diet on long-term cold storage of the predator Podisus maculiventris (Say) (Heteroptera: Pentatomidae)☆
Introduction
One of the primary obstacles to the use of insect natural enemies for biological control is the difficulty and expense of rearing these insects in sufficient numbers and quality for fluctuating markets. More efficient storage methods would substantially improve the options available to insectaries to respond to undulating and unpredictable demands. Cold storage may be a useful technique to enable insectaries and researchers to store insects for extended periods rather than to continuously rear a colony during those times when the colony is not in use. Additionally, cold-storage could assist in accumulating sufficient numbers of insects for innundative releases, or minimizing costs of retaining a colony between inoculative releases, off-season demands and between research studies. Tolerance to cold may also be considered a desirable attribute for shipment and for fitness when used in early or late season releases.
The response of insects to cold temperatures has received considerable interest, with the majority of studies focusing on diapause. However, the role of nutrient quality in diapause is not well understood (Ruberson et al., 1998). The intentional cooling of insects for the purpose of storage has focused mostly on beneficial insects with some of the earlier studies investigating parasitoids (Flanders, 1938, DeBach, 1943). More recently two studies have documented that host quality influences the response of beneficial insects subjected to cold temperatures. In one study involving the predatory pentatomid, Podisus maculiventris (Say) (Heteroptera: Pentatomidae) it was reported that suboptimal food increased the number of individuals that entered a putative diapause condition (Goryshin et al., 1988). In another study, the presence of cryoprotectants in the host were credited with improving the cold hardiness in larvae of the fly pupal parasitoid, Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) (Rivers et al., 2000).
The spined soldier bug, P. maculiventris, is a polyphagous predator of many agricultural pests and is native to North America (McPherson, 1980, McPherson, 1982). P. maculiventris has often been a preferred predator for research, sale and use because of its high reproductive capacity and its ability to be reared on artificial diets. Parallel with interests in storing insects has been a long-established interest in the use of artificial diets to reduce the production costs and maintain stable colonies of insects. As with most beneficial insects, current methods for mass rearing P. maculiventris involve rearing on a natural host. However, several artificial diets have been tested for rearing P. maculiventris, culminating with a zoophytogenous diet that has proven to be cost-effect and has been successfully used to maintain a colony for more than 200 consecutive generations. The zoophytogenous diet has not been fully optimized for continuous rearing and at its original formulation resulted in an increase in developmental time and a decrease in fecundity (Coudron et al., 2002). Other possible effects were indicated by altered gene expression patterns (Yocum et al., 2006, Coudron et al., 2006).
In the present study, the potential of nutrient quality on responses of P. maculiventris to storage at two reduced temperatures were evaluated by measuring survival, fecundity and egg viability of P. maculiventris reared on larvae of Trichoplusia ni or an artificial diet. In addition to Proc Genmod analyses, the use of odds ratios analyses, i.e., an extension of a general linear model analysis, linked to a logit (nonlinear) transformation to provide a good linear fit assigning a binomial distribution, enabled the comparison of uneven data sets that resulted from the extensive differences of each developmental stage in response to the treatments.
Section snippets
Insect colonies and diet
The P. maculiventris colony originated from adults collected in alfalfa in Boone County, Missouri in the summer of 2000 and subsequently maintained continuously at 26 °C, 16L:8D photoperiod, and 75% RH in a walk-in chamber. The prey-fed colony used in this study had been maintained for ca. 80 generations on excess quantities of coddled fourth instar larvae of Trichoplusia ni (Hübner) (Coudron et al., 2000) and the diet-fed colony had been maintained for ca. 10 generations on excess quantities of
Results
At all temperature treatments for nymphs and adults, dead individuals were readily differentiable from live individuals. Although individuals were more lethargic during cold treatment, normal motion and response to touch resumed within 1–2 h after being placed at 26 °C and was used to discern survival. Egg, larval and adult results were compared over those weeks of cold storage where there was survival with each treatment. Adult results were compared over a 10 week period.
Discussion
There was a clear effect of food source quality on egg, nymphal and adult survival, fecundity and egg viability following cold storage. Cold storage treatment of eggs of P. maculiventris held at 4 °C and 10 °C rapidly lowered the percentage hatch even though in this study higher percentages of eggs from prey-fed and diet-fed insects survived storage at both temperatures for longer periods of time (i.e., up to 14 days) than had been previously reported (De Clercq and Degheele, 1993). The results
Acknowledgments
We thank the following ARS employees: Maureen Wright for her extensive contributions to the statistical analyses and manuscript, James Smith and John Willenberg for their creative contributions to rearing and technical assistance in this project and Dr. David Stanley for his critique of the manuscript.
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