Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 26, 2004
Publication Date: January 26, 2005
Citation: Popham, H.J., Shelby, K. 2005. Effect of dietary selenium supplementation on resistance to baculovirus infection. Biological Control 32(3):419-429. Interpretive Summary: When insects feed on plants they encounter many chemical compounds (nutrients), such as Selenium, that they require to stay healthy and to withstand stresses such as insecticide exposure, toxic plant chemicals, and infections. Soil Selenium levels vary widely from location to location, and are concentrated to toxic levels by some agricultural practices such as irrigation. High levels of Selenium are toxic to pest insects and to beneficial insect predators which in turn eat the pests. In humans and cattle, a shortage of Selenium weakens the immune system. In this report we examine whether selenium levels can impact how pest insects fight off infection. Cabbage looper larvae, a pest particularly in greenhouses, were fed increasing levels of Selenium and were then infected with an insect virus. We found that adding selenium to cabbage looper food in low levels makes cabbage loopers more resistant to virus infection. This finding will impact scientists working on resistance of insect pests to virus infection and with more research will impact how growers view selenium levels in crops and how other plants in the field impact biological control agents.
Technical Abstract: Herbivorous insects encounter a range of dietary nutrients, antioxidants, co-factors and plant secondary metabolites which may modulate their resistance to microbial infections. A colony of the lepidopteran pest insect Trichoplusia ni has been maintained at BCIRL for generations on an artificial diet with no added Se. These depleted or low Se, insects grow and reproduce normally. Supplementation of the diet of these Se-depleted larvae with 10 ppm or less Sodium Selenite resulted in no deleterious effects on larval growth. Larvae were reared on three different regimes of increasing levels of Se: 1) Se throughout their larval development; 2) Se depletion until the onset of the fourth instar then repletion of Se; and 3) Se up to the fourth instar followed by Se depletion. Selenium levels of pupae from Se fed larvae showed increasing levels depending on the amount of Se added to the diet while larvae fed Se until the fourth instar displayed the same amount of Se in all groups tested. Larvae reared on the three regimes were infected per os with increasing doses of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). LC50s were not significantly different between control larvae and larvae fed 5 and 10 ppm Se, except larvae fed Se until the fourth instar and then moved to control diet. These larvae had a 5 or 10 fold higher LC50 when fed 5 or 10 ppm Se, respectively. This study indicates that dietary selenium levels do impact the infectivity of AcMNPV in selenium-deprived T. ni.