The Potential Use of Scarab Beetle Larvae in Composting

The Potential Use of Scarab Beetle Larvae in Composting

Liam Foley – Killarney Heights High School

The aim of this paper is to determine whether scarab beetle larvae can be used to help in the process of composting. The experiment was conducted over the course of seven weeks in which varying amounts of Xylotrupes australicus larvae were placed into containers containing either composted or uncomposted substrate. It was found that there was no statistical difference between the growth rate of the larvae in composted and uncomposted substrate. An increase in growth was observed in all larvae regardless of which substrate they were in. Visually, the substrate in the containers that had held larvae appeared to be broken down much finer and contained fecal pellets when compared to the containers that had not held any larvae.

Literature review

The Scarabaeidae family, commonly known as the scarab beetle family is a diverse family consisting of approximately 30,000 species of beetle of which there are over 2,000 species described in Australia (CSIRO). The larvae of scarab beetles have three stages which are known as instars (DPI), and the larvae are usually very difficult to distinguish. Species identification often relies on differences in the pattern of the hairs, called setae, present on the underside of the terminal abdominal segment, called the raster (Timothy Gibb, 2015). Examples of setae patterns can be seen in figure 1.

Figure 1. Rastral setae patterns of four common agricultural pests (Timothy Gibb, 2015)

The immature stages of many species feed solely on decaying organic matter and animal faeces. However, other species feed on decaying organic matter and living plants; generally roots and/or basal stem tissue (GreenLifeIndustry, 2017). Such species are pests that can cause serious damage. In natural ecosystems, beetles and their associated microorganisms perform important functions as prime contributors to the degradation of organic matter (K. RojasJiménez, M. Hernández. 2015). Due to

the difficulty in distinguishing the larvae of pest species from other species, this often leads to the indiscriminate killing of native species. This is further exacerbated in the gardening community where misinformation often leads to the recommendation that any beetle larvae should be eradicated. I have frequently observed this advice being given in the Facebook group, “Gardening Australia”, as well as several other gardening related groups. Through conducting my experiment I want to explore a potential alternative to indiscriminately killing scarab beetle larvae. There are several species of scarab pest species in Australia, one of the most well known ones being Heteronychus arator, commonly known as the African black beetle. H. arator is a common agricultural pest, Larvae prune or completely sever roots of perennial grasses and in severe cases where larval populations are high, pasture becomes patchy and can be rolled back like a carpet. (Cesar, 2015). Several studies have been conducted on these agricultural pests in order to gain a better understanding of their feeding habits and preferences. In a 1977 study titled, “Effect of plant species and organic matter on feeding behaviour and weight gain of larval black beetle, Heteronychus arator (Coleoptera: Scarabaeidae)” conducted by P. D. King looked at the effect of several plant species on the movement and feeding behaviour of H. arator and the influence of organic matter on those responses. The results of the experiment were determined using choice chamber testing over a 21 day trial period, repeating the test until fifty larvae had been tested. They monitored weight gain, movement and the number of fecal pellets produced by each larva. In his results he highlighted that organic matter stimulated feeding and increased weight gain (P.D. King, 1977).

Household compost includes several organisms bringing about the whole process of composting. It has its own micro flora-fauna and macro flora-fauna which include scavengers, predators and decomposers. Although the role of other organisms is noticed, there is a lot of emphasis on either microorganisms or earthworms, not much attention is given to other macro organisms often present in compost (Gayatri. A et Al, 2009).

Scientific Research Question

Will Xylotrupes australicus larvae grow more in composted material or uncomposted material?

Scientific Hypothesis

The larvae will grow more as a result of feeding on the composted substrate.

Materials and Methodology

Two types of substrate were used in the experiment. The first substrate was homemade compost from a garden compost bin as shown in Figure 2.

Figure 2: Homemade compost

Figure 3: Uncomposted substrate prior to being mixed

The second substrate was a mixture of finely chopped uncomposted organic matter. The carbon rich components of this substrate were dried leaves, sugar cane mulch, cocopeat and were mixed in a ratio 2:3:1, and the nitrogen rich components were a random mixture of finely diced household vegetable scraps (such as leafy greens, potato peels, and carrot tops). The carbon-rich and nitrogen-rich components were mixed in a ratio of 25:1 as recommended by the Plant Natural Research Centre website, as shown in Figure 3. 1 part store bought compost was also added to the mixture in order to aid in the composting process by adding the appropriate microorganisms.

The experiment was conducted using 8 300ml plastic containers with ventilation holes added to the lids. Half of the containers were filled with the compost, while the other half were filled with the uncomposted mixture. 75ml of water was added to each of the containers containing the uncomposted substrate. Varying numbers of small Xylotrupes australicus larvae were weighed and then added to the containers, as shown in Figure 4 below, which were labeled with a reference indicating composted or uncomposted and the number of larvae. These combinations are represented in Table 1 below.

Figure 4: Containers with varying numbers of larvae

Composted Uncomposted

0 Larvae C0 U0

1 Larva C1 U1

2 Larvae C2 U2

3 Larvae C3 U3

Table 1: Container Labeling

All containers were placed in a cupboard to remain undisturbed for the duration of the experiment with the exception of weekly weigh-ins.

On a weekly basis the containers were removed from the cupboard and the larvae in each container were weighed individually using a I-2000 digital platform scale provided by KHHS, with a stated accuracy of ±0.01g.

Results

All measurements were taken in grams and recorded to two decimal places, presented in Table 2 below. Note the reference to “random” is to highlight the fact that the weights recorded in a column

are not necessarily for the same larva cells in the table. The deceased larvae with each sample. Additionally some were not included in the calculations for larvae died and this is indicated by grey averages.

Table 2: Raw data.

An analysis was generated on the raw data, to calculate the average weight of the larvae in each container, this is shown in Table 3

Table 3: The average weight of larvae by container over time

This data was then used to calculate the average weight of all the larvae each week, based on substrate type as shown in Table 4. This data is represented in Figure 5.

Average Weight (g) of Larvae by Substrate Over Time Series

1 2 3 4 5 6 7

Uncomposted 0.965 1.030 1.083 1.140 1.332 1.388 1.470 Composted 0.664 0.755 0.848 1.000 1.168 1.387 1.552

Table 4: The average weight of larvae by substrate over time

Figure 5: Average weight of larvae over time

A T-test was conducted to see if the difference between the two means of weight of the larvae in uncomposted and composted substrate was significant at ����<0.05, refer to Table 5.

Table 5: T- Test

Discussion

While all larvae gained weight, and the graph in Figure 5 appears to indicate an increased growth rate for the larvae in the compost substrate, by doing a t-test I determined that I could not reject the null as my p-value was greater than 0.05. This meant that there was no statistical difference between the growth rate of larvae in the composted and uncomposted substrate. My null hypothesis was that the larvae will not grow more as a result of feeding on the composted substrate. Upon visual comparison of the containers containing larvae with those that did not house any larvae (U0 and C0), the matter appeared to be broken down much finer and contained fecal pellets. Although there is no statistical difference between the growth rates, it is clear that there is a general increase in the weight of the larvae in both substrates over time. There was a 52% increase in the average weight of larvae in uncomposted versus a 134% increase for larvae in the composted substrate. This indicates that there is potential for scarab beetle larvae as composting organisms as they grew even when feeding and therefore, by implication, helped break down material in substrates at two drastically different stages of decomposition. There were a number of factors that affected the results of my experiment, the main one being temperature. As X. australicus is a tropical/subtropical species mainly found in NE NSW and QLD, they are not suited to Sydney's temperate climate, especially in winter which is where and when the experiment was conducted. As they received no artificial warmth, their metabolism would have been much slower and therefore they would have consumed less and as a result grown less than if the experiment had been conducted in summer when the temperatures are warmer. The cold temperatures may have also been the cause of death of the three larvae that died during the experiment. This is likely because first instar larvae are much less tolerant of cold temperatures. The limitations of my experiment also included the limited range of species, the size of the containers, and the quality of the substrate. If I were to repeat the experiment I would use a local species of scarab beetle and I would conduct the experiment in summer when it is warm, I would also use third instar larvae as they are more hardy than first instar larvae, they consume more food than previous stages and gain more weight (GreenLifeIndustry, 2014). Further experiments could be conducted to test the fertility of compost broken down by scarab beetle larvae versus that that hasn't. This could be done by measuring the growth of plants grown in compost which has been broken down by scarab beetle larvae versus compost which has not been broken down by scarab beetle larvae. Future research should be done on the diets and feeding habits of native scarab species in order to determine which species may act well as potential composters.

Conclusion

After conducting a t-Test it was determined that there was no statistical difference between the growth rate of larvae in the composted and uncomposted substrate, this is because the p-value was over 0.05. It was demonstrated that X. australicus larvae

will consume and grow in either substrate. A visual comparison of the containers containing larvae with those that did not house any larvae (U0 and C0), revealed that the matter appeared to be broken down much finer and contained fecal pellets. I determined that there is potential for scarab beetle larvae to be used as composting organisms as

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