Curtis et al., 2009, say that J2 of Meloidogyne species, hatch when environmental conditions, such as appropriate temperature, oxygen availability and moisture levels, are suitable; and there is an absence of physiological barriers, such as diapause.

Guiran and Villemin, 1980 () demostrated that certain species of Meloidogyne exhibit diapause, although the percentage of unhatched J2 in diapause varies considerably from 10% in predominantly tropical M. arenaria, to 94% in M. naasi, which has a more temperature distribution.

Hatched J2 are more vulnerable to environmental stresses than if they had remained unhatched. In addition to the protects that affords the gelatinous matrix, the eggshells has three layers of protection: an outer vitelline, a middle chitinous and an inner lipid. Inmediately before hatching, the changes in the eggshell are manifested by a marked change in permeability. The unhatched J2 produced proteins that erode the layers of eggshells, prior tho hatching.

Hatching of J2 is primarily temperature driven. For exemple, species cryophili can hatch at temperature below 10ºC, whereas thermophilis do not hatch at temperatures below 15ºC.

For Meloidogyne whit relatively wide host ranges, the influence of exudate is less apparent, with most J2 hatching in water without the need for exudate simulation. However, some J2 withing a population, depend on root exudate to initiate the hatching process. The exudate affect the rate of hatch of this species.

Perry, Moens and Starr, 2009.Root knot nematodes. 

See work by Roland Perry.

Interesting question. Thus your study DIAPAUSE, where the RKN eggs are one of many forms The stages in a diapause are the golden deposits of the population, like the cell in a bank. Here may ne different stimuli for hatching, one of them is TIME.

As far as experienced by myself, Meloidogyne chitwoodi and M. hapla are quite easily hatching in water. From one tomato plant 1 million juveniles can be obtained within 10 days from a spray-mist chamber. Those eggs that do not hatch are embryonic eggs. Furthermore, there is a possibility to stop eggs from hatching by adding a small amount of sugar(?) to the water. Embryonic eggs will develop into mature eggs but will not hatch. This method was developed by a breeding company in The Netherlands (HZPC) to make sure all eggs would hatch at the same time when they were needed. I am not 100% sure it was sugar they added.   

" This method was developed by a breeding company in The Netherlands (HZPC) to make sure all eggs would hatch at the same time when they were needed"  It was not sugar, it was NaCl.

also don't forget that some times no hatching could mean no eggs in the egg masses (if for some reasons female nematode established in the roots but have a fertility problem; this can be tested easily by extracting the eggs from the roots).

Thank you all for your valuable comments. I benefited a lot from them.

Yes oxygen availability affect egg mass along with moisture availability, temperature and time.

all mentioned above can effect, as our experience time, temperature and moisture is important for RKN egg hatching.