Mormon crickets

Radio Tagged Mormon crickets

Mormon crickets (Anabrus simplex; Orthoptera: Tettigoniidae) are fascinating for many reasons, but two have guided my recent ecological research. First, these wingless katydids exhibit dramatic variation in mating behavior across southwestern North America. In parts of their range, they are typical polygynous insects (green animal pictured). Males compete for mates, females choose among males. In other parts of their range, typical sex-roles are reversed (dark brown example). Females compete for mates and males discriminate among females. This dramatic variation in mating system is strongly correlated with nutrient limitation. Where nutrients are scarce, sex-roles are generally reversed. However, nutrient limitation, while necessary for sex-role reversal, is not sufficient to explain its occurrence. There are many other animals (e.g., grasshoppers), with similar ranges and no sex-role reversal. Reversal seems to depend critically on another component of Mormon cricket ecology correlated with nutrient limitation. This is the second fascinating thing about Mormon crickets --- that they sometimes are found at extremely high density in fast-moving groups. During Mormon cricket ``outbreaks'' (very high densities occurring for several years), Mormon crickets form large cohesive groups called migratory bands. These bands can contain millions of insects (> 50/m2) that have been observed to travel en masse up to 2 km per day.

Here is a one minute video of Mormon crickets on the move. (Click on the image to download a 4 Megabyte Quicktime movie. Rendition of "Band on the Run" by The New Duncan Imperials)

Band on the run

Understanding what causes these dense bands and their movement is one of my main research interests. In addition to insight into mating system evolution, this research can also give more general insight into insect outbreaks and mass movements. The eventual goal of this research is to be able to predict Mormon cricket density and movement. Knowing what controls the rate and direction of moving, dense groups of insects is interesting for two reasons. First, this knowledge will allow more effective control of Mormon cricket outbreaks and insect outbreaks in general. For example, being able to predict where banding Mormon crickets will go next will allow managers to better prevent crop and rangeland damage while reducing environmental and financial costs of control (millions of dollars across several western states). Second, because high-density bands of Mormon crickets are both very unusual (other insects in these areas do not do it) and devastating to local vegetation, understanding "banding" will increase what we know about the community ecology of outbreaks. Specifically, we will better understand what ecological factors contribute to outbreaks and mass movements, and we will better understand how these moving outbreaks affect the community in which they occur.

Another topic of interest that we have very little data on is what keeps bands moving in a consistent direction over days. Movement of a small number of crickets through the night may provide an answer. Cricket movement has always been assumed to stop at night. At least some individuals form dense "creches" with hundreds of crickets, densely packed into a stationary mass.

Very little is known about why Mormon crickets are found at high density in some years, nor why outbreaks only seem to appear in the dryest portions of their range where nutrients are relatively poor. Mormon cricket eggs diapause in the soil where they are laid by females as they march, so the eggs themselves are not likely to be clumped until band densities are already very high. The eggs may diapause in the soil for many years, hatching only in response to a complex set of cues, as yet not understood. This complex diapause has the potential to synchronize hatching so that nymphs emerge together, but again this seems unlikely to explain high densities until band densities are already high.

Plughat site map

In conjunction with Greg Sword, Darryl Gwynne, Stephen Simpson and Ian Couzin, I am pursuing several avenues to better understand how and why high density bands form and move. We began by describing the basic differences in density and movement between the two extreme population types using 0.5 g radio transmitters (see first figure for picture of radio and second for example of tracking data). During 2004, in replicated transplant experiments in the field, we examined the relative importance of environmental as compared to social cues for band cohesion and movement. Preliminary results indicate that "selfish herd" effects may explain both high density and movement. We found in each of three replicates that when band members are transplanted to comparable habitat with no other crickets, 60% are killed by predators while none of the comparably radio-tagged and handled animals left in the band are killed. This dramatic result suggests that advantages to being at the center of a group may explain both high band density and band movement. Animals on the band's periphery would be most vulnerable to predators and should move into the center of the band. Any asymmetry in this movement toward the center could start the band moving in a particular direction. Future work needs to be done to test this hypothesis for why bands move. We know from previous work that movement is consistent over days and that individuals that are 20 to30 meters apart often turn in the same direction. These two observations are difficult to explain by selfish herd effects alone.

In 2005 we were able to experimentally manipulate diet and hunger levels and show that movement may be occurring to get nutrients that are in short supply.  Mormon cricket may also move to avoid cannibalism.  Marching crickets are short of protein and salt.  When they are nutrient deprived they walk more and are more likely to cannibalize.  Taken together nutrient deprivation and fear of cannibalism may motivate crickets to move in a coordinated way.  We now know both an important reason why Mormon crickets join bands and that there are costs to band membership that may be reduced by movement.

Animation

Tracks and animation generated using ImageJ and the Manual Tracking plugin by Fabrice Cordeli�res, Institut Curie, Orsay (France).