When people talk about fungi it’s usually in the context of a tasty dish, or that fuzzy brown thing on your peach, but rarely ever are fungi referred to as deft, vicious killers. What you say? Killers? Well, fungal pathogens have long preyed on insects, claiming millions of victims yearly and all over the world. An array of pathogenic fungal genera can infect a broad range of insects [and spiders! --Ed.]. Once an insect has been exposed to spores, the developing fungus may induce behavioral responses in its host. These behavioral responses may result in improved spore dispersal and fungal fitness. But behavioral responses may also be defensive, ultimately benefiting the host. One of these responses is behavioral fever, in which insects raise their body temperature as a means of literally toasting a fungal invader.
Human immune systems react to an invader by raising the internal temperature (you get a fever). Insects are ectotherms (cold-blooded), so to acquire a fever, they must increase their temperature via behavioral means. Many insects will climb up a plant stem to get closer to the sun or to some other heat source in the lab. In Denmark, a field study was staged in an empty barn that had heat lamps and different temperature areas. Flies were inoculated with the deadly pathogen Entomophthora schizophorae, marked, and set free. A significantly higher number of marked flies congregated around the heat lamps compared to the uninoculated (control) flies. This suggests that the infected flies were actively trying to raise their body temperature in response to infection (Kalsbeek et al 2001).
In another study, grasshoppers were inoculated with the fungus Beauveria bassiana, a common insect pathogen with a wide range of hosts. The unfortunate grasshoppers were housed in a cage with a light source mounted on one wall. Each test group was either not allowed to bask in the light, or was given the option to bask for a specified amount of time each day (Inglis et al. 1996). The authors found that “in nymphs permitted to bask for only 1 h per day, 46% less mycosis was observed, and as the basking period increased, so did the inhibition of the disease.” In this study grasshoppers were able to cure themselves from mycosis through thermoregulatory behavior (Inglis et al. 1996). A different study with locusts infected with Metarhizium anisopliae var. acridum used a thermal gradient that spanned 28-50°C. The survival rate of infected locusts was still significantly less than the uninfected (control) locusts, but the infected locusts all showed a preference for higher temperatures (Blanford and Thomas 1999). More insects survive fungal infection when allowed to warm themselves, so a behavioral fever response is clearly advantageous.
The reason behavioral fever can be effective is probably related to fungal optimum growth temperatures. Each fungal species or isolate has a range of temperatures in which it grows most aggressively. Insects like locusts can exploit this weakness by elevating their body temperatures from their optimum of 38-40°C to a steaming 42-44°C in response to an immune attack. The heat is thought to boost the host immune systems and retard fungal growth (Eliot et al. 2002). In the case of locusts, the fever was not enough to for a cure–the main benefit was prolonged survival while infected. As soon as the thermal gradient was taken away, they quickly succumbed to infection (Eliot et al. 2002). To agriculturists, this insect defense is a nuisance because behavioral fever can slow down the killing power of a biocontrol pathogen. When fungal pesticides are used, results are varied and delayed, much to the dismay of farmers.
The field of biocontrol is a burgeoning industry aimed at a more focused, less harmful approach to pest management. This industry has recently homed in on the usage of fungal pathogens to control pest insect populations, specifically agricultural pests (Hajek 2004). Who knew that such an awesome system existed in nature? To the disappointment of many, these fungal systems have been only moderately successful in most cases. Often the reason for failure is related to the ambient temperature of the test site. It has been shown that fungal spraying during cool overcast days is more successful at eliminating insects than spraying during a hot and sunny season (Inglis et al 1994). This observation indirectly supports the importance of insect behavioral fever, which can only be effective if the environment provides a means for the insect to raise its temperature (through the sun). Other problems in the effectiveness of biocontrol often revolve around the speed of the fungal attack. When ambient temperatures fluctuate, warmer temperatures may limit the speed at which the fungus can take over its host. Nevertheless, the biocontrol industry has found ways to get around the effects of warmer temperatures on fungal pathogenesis through the use of more heat tolerant strains of some fungi (Hajek 2001).
- Blanford, S. and Thomas, M.B. 1999. Host thermal biology: the key to understanding host-pathogen interactions and microbial pest control? Agricultural and Forest Entomology 1:195-202.
- Elliot, S.L., Blanford, S., and Thomas, M.B. 2002. Host-pathogen interactions in a varying environment: temperature, behavioral fever, and fitness. Proceedings of the Royal Society B: Biological Sciences 269: 1599-1607.
- Hajek, A. 2004. Natural Enemies: an introduction to biological control. Cambridge University Press (UK).
- Inglis, D., Johnson, D.L., and Goettel, M.S. 1996. Effects of temperature on thermoregulation on mycosis by Beauveria bassiana in Grasshoppers. Biological Control 7: 131-139.
- Kalsbeek, V., Mullens, B.A., and Jespersen, J.B. 2001. Field Studies of Entomophthora (Zygomycetes:Entomophthorales) — Induced Behavioral Fever in Musca domestica (Diptera: Muscidae) in Denmark. Biological Control 21(3):264-273.
Photos: Jodi Creasap (Entomopthora on fly); K.T. Hodge (Beauveria amorpha collected in Uganda)
Happy New Year, all!