COCKROACHES, ANTS AND OTHER SMALL BEASTIES – THEIR IMPORTANCE IN MULGA COUNTRY

December 6, 2024

Tony Pressland, Ross McIntyre, Don Cowan and John Lehane (ex Department of Primary Industries, Charleville Pastoral Laboratory, Queensland). Email: apressla@bigpond.net.au

What this is about?

Whilst undertaking soil water measurements in the mulga (Acacia aneura F Muell.) country in Queensland in the 1970’s (see Figure 1), we found some important insect activities which have great potential to influence water infiltration into the soil. The holes made by ants, insect larvae and the giant cockroach, Macropanesthia rhinoceros, were very important in allowing rainwater to infiltrate mulga soils which have low organic levels and high levels of aluminium and iron oxides and hydroxides which reduce the ability of soils to take in water. These insects also have a role in breakdown of litter and mineral cycling.

Figure 1. Mulga (Acacia aneura) country in south western Queensland

The cockroach (Figures 2 and 3) appears to be only observed in reasonably dense mulga, eg ~ >500 trees/ha. It does not appear to be associated with denuded/degraded mulga or where it is sparse. The cockroach spends much of its life underground, emerging at night or when atmospheric conditions are favourable (during significant (>12mm) rainfall events, though falls in excess of 150 mm in the summer of 1971 did not produce much activity). The cockroach digs hemi-circular tunnels up to about 60 cm into the soil. Some tunnels can be quite long – one we excavated was just over a meter in length. At the end of the tunnel is a small chamber which had been partially filled with mulga phyllodes – virtually a nest. This movement of leaf from the surface to some depth in the soil would be instrumental in aiding nutrient cycling as well as improving the aeration of soil due to an increase in organic matter.

Figure 2 . The giant cockroach Macropanesthia rhinoceros. Photo – Flickr

Figure 3. The giant cockroach. (Scale is in inches – 1in = 25mm). Photo – R. Silcock

On emergence from the ground, this cockroach leaves a raised area of soil about 30 cm in diameter and 5 cm high (Figures 4 and 5), which would aid not only in infiltration of rainwater, but also help incorporate litter into the soil – soil notoriously low in organic matter. Tunnels dug by these insects could be really important in infiltration if they were present in sufficient numbers. We did a small study of this.

Figure 4. The mound of soil made by the cockroach in digging the tunnels into the soil. Note the height (~5 cm) and area (~30 cm diameter) of the mound. Photo – R. Silcock

Figure 5. Area of mulga showing mounds made by the cockroach in the soil and young mulga growing profusely. Photo – R. Silcock

What we did

We used a tractor blade to remove the surface 2 cm of soil from a radius of 5 m around two mulga trees. The trees were mature with a trunk circumference measured 30 cm above ground of about 60 cm and a height of about 6 m. We marked out concentric circles at a distance from the base of each tree of 0.5, 1, 2, 3 and 4 m. All cockroach holes as well as those made by ants (species not identified) and insect larvae (mainly Scaraboeidae) were counted in the 0-0.5, 0.5-1, 1-2 and 2-3 m bands of one tree, and the 0-0.5 and 0.5-1 m of tree 2. Holes present around tree 2 in the 1-2, 2-3 and 3-4 m bands were estimated using 8 randomly placed 0.5 m² quadrats.

Soil moisture to a depth of 135 cm was recorded in nearby trees. The period we used for this study followed 50 mm of rain in May 1972. Soil moisture was measured at distances of 0.5, 2 and 4 m from the base of trees. Three replicate samples were taken at 15 cm intervals on four occasions between 12 May 1972 and 22 June 1972.

Water infiltration into the soil was measured using double-ringed infiltrometers – 30 cm outer ring diameter, 20 cm inner ring. Successive increments of rain water, in volume equivalent to 25 mm rain, were applied to the rings. The time taken for disappearance of each successive increment was noted. Infiltration was measured in three replicates at four distances from mulga trees – 0.25, 0.5, 1 and 4 m.

What we found out

The presence of insect holes is shown in Table 1 and the soil moisture in Table 2. Insect holes tended to be highest closest to the tree trunk. There were large numbers of ant holes. Cockroach holes were mainly found within one meter of the tree trunk. This is the area that stemflow from mulga infiltrates (Pressland 1973), so the role of insects in aerating the soil could be considerable. Stemflow averages 23% of incoming rain in mulga woodland (Pressland 1973).

Soil water tended to be higher at depths > 45 cm close to the trunk, particularly shortly after rain (12/5/1972). With time, moisture was reduced quite quickly at depths > 45 cm. Stemflow and throughflow tend to result in a fairly definite soil moisture pattern under a mulga woodland (see Pressland 1975). There is one downside to the depth to which water infiltrates the soil close to mulga – there is a possibility there is a possibility of deep drainage out of the root zone particularly of grasses, but also trees. The deeper, sandier mulga soils may be more at risk of this occurring. However, the frequency of this happening is probably low in the semi arid and arid areas where mulga exists, but with climate change, episodic high rainfall events appear to be on the increase.

Table 1. Stratification of insect holes from the base of two mulga trees

Table 2. The effect of distance from tree trunk on soil water content (%)

Water infiltration (Figure 6) shows that infiltration of water close to the tree trunk (0.25 and 0.5 m) occurred much faster than further away (1 and 4 m). The reason for the slow infiltration for the first three increments of 25 mm equivalent is not known as the site was not excavated after measurement.

The importance of holes in soil made by insects should not be underestimated. Our data shows that holes facilitate the infiltration of water into the soil, and allow water to reach a greater depth in the soil. There is a close relationship between number of holes in the soil and distance from trees (Table 1); ability of soil to accept infiltrating water from stemflow (Figure 6); and the depth to which water many infiltrate the soil (Table 2). Stemflow in mulga can be considerable; for example, a fall of 25 mm can produce 200 L of water down the stem of a well shaped mulga tree with a height of about 6 m, a trunk circumference 30 cm above ground level of about 60 cm and a canopy area of about 22 m².

Without insect holes, infiltration is limited (Fig 6) and soils moisture levels lower, inferring higher runoff. This can be the cause of erosion, loss of surface soil, and reduction in ground cover by vegetation. Further, water infiltrating to greater depths may allow mulga to produce flowers and seeds more often than in the absence of stemflow or water getting to depth.

Figure 6. Water infiltration into the soil at distances of 0.25, 0.5, 1 and 4 m from tree trunk

Acknowledgements

The photographs (Figures 3, 4 and 5) were kindly provided by Richard Silcock. Figure 2 is from Giant Burrowing Cockroach (Macropanesthia rhinoceros) | Flickr.

This study was undertaken as part of many funded by the Australian Wool Board.

COCKROACHES, ANTS AND OTHER SMALL BEASTIES – THEIR IMPORTANCE IN MULGA COUNTRY

What this is about?

What we did

What we found out

Further reading

Acknowledgements

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