While Henry Dybas's work is very good and interesting, it does represent a good start to deal with terrestrial patterns where insect density is related to the actual space they live.  The locust swarms can be very dense in local areas (especially if we consider them when flying, a 3-D density), but of course they are moving and using the resources over a broad area.  Another even denser type terrestrial mass of insects is the emergence of mayflies.  They are so dense, that even the weather service can monitor the swarms using radar (see story link below).  If you google old pictures of mayfly emergences you can see bridges covered to several feet in depth with mayflies, ranging in the 100's of millions.  If you find a review or want to write one, please let me know. 

https://www.usanpn.org/fws/MayflyWatch

Thanks for reminding me about mayflies, Robert. I experience these swarms every year in my hometown on the Mississippi River. According to http://www.usgs.gov/newsroom/article.asp?ID=1301#.Vxjs8_krK70, Mayflies used to reach 'several' 100s per m2, which equates to a few million per ha.  

Some insects can reach very high densities during their periodic outbreaks, e.g. forest insects such as some Coleoptera and Lapidoptera. I remember an outbreak of the leaf beetle Chrysomela aenea on Alnus incana, that I observed along a stream in Italian Alps. I don't have numeric data, but the density of adult insects (as well as preimaginal stages) on each leaf, on each tree, with an impressing degree of defoliation,  was extraordinary. Perhaps the book by P. Barbosa: Insect Outbreaks, is useful. Best regards,

Rinaldo Nicoli

A paper (url below) found 630 mayflies per square meter at one site, so that would be 6.3 million per hectare.  I would still like to find info on the density of the emergences when they pile up several feet thick - think this would qualify as a geologic phenomena.

http://link.springer.com/chapter/10.1007%2F978-94-009-2397-3_27#page-1

I don't have any actual numbers for you, but I would be surprised if the highest densities are in any group other than aphids, scale insects or ants. Muc

I think you may be correct, and I am sure we can find some article on ant density.  Ant density would also be a more stable measure of density, while locust swarms are so temporary as are the huge piles of mayflies after an emergence (but their numbers are impressive).

Sorry, looks like my cellphone was too eager to have this posted. I was trying to say: much though some bigger insects may aggregate temporarily in huge numbers, the body size effect must be overwhelming. Having thought a bit longer, I would add a spatial scale effect. Over square centimetres and also over square kilometres it must be very small insects that are most numerous. But maybe over square metres or so, for short periods of time, locusts or mayflies may prevail.

  I  recommend you that the outbreak of Brown rice planthopper (Nilaparvata lugens(Stal)) in Chinese rice growing area in 2005. It may be an interesting case for you.

The question of Wayne is very precise. What are the densest insect species in the world? Springtails  (Collembola) are unquestionably the most abundant insects in the world. An average square metre of soil can yield as 200,000  individulas, so that would be  2.000 million per hectare, in other words 2.000.000.000 millions of individuals/ha. These tiny insects are surface dwellers hardly 5 mm long, and the subterranean forms often barely reach 1 mm in length.

Regards,

Luis Miguel

Are we talking just about individual density? Then somebody should look for published data about collembolans. If we talk about epigeic groups or species, one should look for published data about diptera hatching from soil and sampled in some kind of tent traps. Next I can offer 4.500 Auchenorrhyncha per sqm (= 45.000.000 per ha) sampled with a suction sampler (Nickel & Hildebrandt 2003), but there might be some slightly higher counts in this group. Unlike some other data on outbreaks or mass movements within other groups, Auchenorrhyncha densities between 1.000 and 5.000 per sqm are at least locally "normal" and widespread in grasslands. Concerning insect biomass per sqm 17-year cicadas shortly before hatch might be on top, though only patchily.

@Luis: Collembola are not insects according to the latest classifications. But as hexapods, yes, they may take the cake.

Then, How about Hymenopterans? We have an unpublished data of honey bees ranging 40,000 / 10 meter square. Is it the correct estimate if converted to population per hectare or so? Pl. see a part of the tree with many number of honey combs with bees.... More than 20 such combs are found during summer season in a single tree in Southern Eastern Ghats, Tamil Nadu, India. 

Català, et al, (http://www.irta.cat/ca-ES/RIT/Noticies/Documents/prod_vegetal_articlequironomics1-baja.pdf) reports 10.000 Chironomid larvae/m2 in rice germinating in Delta del Ebro. = 10.000.000.000 ind/ha.

 Sorry = 100.000.000 ind/ha

Jonathan, Do you have a link you could share for the articles that were published?