ARS | Publication request: Iron levels change in larval Heliothis virescens tissues following baculovirus infection

Submitted to: Biological Trace Element Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 21, 2012
Publication Date: July 19, 2012
Citation: Popham, H.J., Sun, R., Shelby, K., Robertson, D. 2012. Iron levels change in larval Heliothis virescens tissues following baculovirus infection. Biological Trace Element Research. 148(3):356-362.

Interpretive Summary: Dietary iron is an extremely important nutrient required by animals to, among many other things, fight off fatal infections. But pest insects also require iron and may actually benefit from iron biofortification intended for humans and livestock. Therefore it is important to determine whether iron levels present in crops help or hinder the usefulness of an insect virus used as a biological control agent for an insect pest. Very little is presently known about the iron requirements of pest insects, thus we conducted the first study of blood and tissue iron levels in budworm caterpillars following infection with two viral biological control agents. Viral infection of caterpillars greatly increased the blood levels of iron, especially in the late stages of infection. Trace labeled iron fed to the budworms was taken up from the gut into blood and then stored in internal organs. Upon infection with baculoviruses this stored iron was released into the blood bound to proteins. This was the first study of the role that iron plays in the biological control agent baculoviruses. This finding will impact scientists working on how an insect resists infection because it begins to explain, along with other studies in the field, why some infections are fatal while others are not and how baculoviruses can be made more useful as a control measure in the field.

Technical Abstract: Inductively-coupled plasma mass spectrometry (ICP-MS) and 59Fe radiotracers were used to investigate changes in levels of iron (Fe) in the tissues of Heliothis virescens following baculovirus infection. Fe concentrations were determined by ICP-MS in hemolymph collected from 4th instar larvae infected with Helicoverpa zea single nucleopolyhedrovirus or with Autographa californica multiple nucleopolyhedrovirus. Baculovirus infection led to significant changes in hemolymph Fe levels late in infection. In a series of pulse-labeled radiotracer experiments, 24Na tracer fed to 4th instar larvae was not retained in the insect and reached nearly undetectable levels 6 h post ingestion. In contrast, 59Fe radiotracer fed to 4th instar larvae declined within the first few hours of ingestion and then remained constant at approximately 60% of the initial tracer activity. While Fe radiotracer levels among larval tissues changed, the overall whole insect tracer levels did not decline from 6 – 60 h post ingestion. In a comparison of tissues from HzSNPV and uninfected larvae, those from baculovirus infected larvae had higher radiotracer levels in the hemolymph and midgut at both 36 and 60 h post infection. A significant difference was found in the bound/free ratio of 59Fe (large protein bound Fe: small protein bound and/or free Fe) between control and baculovirus infected hemolymph at 60 h post-infection indicating that Fe released from damaged cells is taken into hemolymph not as free Fe. In both the ICP-MS and tracer studies Fe levels were higher in the plasma/hemolymph of HzSNPV infected larvae than in uninfected larvae. This is the first study of tissue Fe levels during viral infection of an insect, and clearly demonstrates that larval Fe homeostasis is substantially disrupted by baculoviral infection.