Our results suggest a significant negative impact of hematophagous flies, on cattle annoyance, health and growth.
4.1 Discussion
Fly densities, peculiarly hematophagous flies, were quite variable during the two experiments; however, in both cases, they exhibited two serial peaks, which induced fluctuations in the impact-parameters recorded. Overall, fly densities were considered as high (> 400 hematophagous flies /trap/day) during half of Experiment 1 (weeks 4–5 and 8–9) and Experiment 2 (weeks 4–14 and 18–22); they were lower in other periods. The impact recorded on cattle might then be limited to a half of the experimental periods, thus, higher impact may be expected in case of higher or more constant fly density.
Direct impact of flies on cattle behavior was obvious from the tail-pedometer records (TPRs), with, on average, more than 12 times more “steps” recorded in Group B (mean 24 hours TPR 8,879 ± 3,545) versus Group A (736 ± 91) in Experiment 2; however, a part of these defense movements were due to nocturnal insects as discussed below, when splitting diurnal and nocturnal TPRs.
Beside the tail flicks, the feed intake could possibly be affected by the flies’ annoyance, however, in the conditions of our experiments, the feed intake was not significantly different in the two groups, both in terms of feed-ingestion rhythm and total weight of feed and dry matter ingested. However, the feed conversion ratio (DMI/weight gain), was higher in the exposed group, with a strong statistical significance in Experiment 2 (Group B needed 133% of the DMI needed by Group A for a similar weight gain), allowing to conclude to a decreased feed-efficiency induced by the insect’s annoyance. Similar results had been previously obtained in 4 ways mixed cattle feedlots (mean weight of 272 Kg at the beginning of the experiment) under the pressure of natural Stomoxys spp. population in Nebraska (Campbell et al. 1987).
Due to blood cells ingestion, hematophagous flies are expected to impact the hematocrit of exposed cattle; indeed, a significant impact was recorded in groups B, especially in Experiment 2 (lasting longer than Experiment 1), where the mean PCV of exposed animals reached a value 3–4% below that of the control group, synchronized with the high fly density periods (in Experiments 1 and 2), and showing an immediate direct effect of hematophagous flies on this cattle health blood parameter.
The live body weight gain (LBWG) of cattle was also impacted by high fly densities, as shown by the highly significant results obtained when comparing LBWG of groups A and B. In Experiment 2, the mean loss of LBWG of group B was of 8.0 ± 1.5 Kg/month [2.7;13.3]. This loss being recorded under a “medium” fly pressure (apparent density per trap < 500 in both experiments), greater losses can be expected under higher fly pressure.
A direct link between LBWG and flies density is not easy to draw, since other parameters are impacting weight gains, such as compensatory growing after a period of reduced growing, habituation of the cattle to high fly density after a period of exposure, climatic conditions, interactions between cattle in a group etc. However, the global impact of flies on the weight gain measured in this study is calling for a serious attention in terms of loss of incomes. In order to use a universal unit, we calculated the shortfall of the weight gain on feeder cattle, taking as a model the results of Experiment 2. In this experiment, within 20 weeks of exposure to flies (weeks 3–22), the loss of LBWG of Group B reached 40.0 ± 5.5 Kg. This loss of LBWG amounted 10.6% of the LBW of exposed cattle, lost within five months, during their main growing period (10–15 months of age). When considering the current value of KPS beef LBW in Thailand (estimated around 125 THB/Kg or 4 USD/Kg in 2019), a feedlot of a 100 calves would amount a total loss of around 4 tons of LBW, for a value of $16.000.
In a study carried out in the US, based on the cattle inventories and average prices for 2005–2009, and median monthly infestation levels, national losses were estimated to be $360 million for dairy cattle, $358 million for cow-calf herds, $1,268 million for pastured cattle, and $226 million for cattle on feed, for a total impact to U.S. cattle industries of $2,211 million per year. We would hardly extrapolate our results to a whole country, but, depending on farming conditions, the economic impact of flies might be very serious, and could be roughly estimated around 10% of the cattle LBW, lost during a five months period of medium to high flies density. Our results are quite consistent with previous estimations made in the US (West Central Nebraska), in which, on average, losses of weight gain recorded in grazing yearling cattle exposed to Stomoxys calcitrans were estimated around 0.2 Kg/day (Campbell et al. 2001); extrapolated to a period of five month, the loss would amount to a total of 30 Kg of LBW/head. Our results (a total loss of 40Kg LBW/head) being obtained only over a 5 months period, it is possible that the longer observation made by Campbell et al somehow “buffered” the effects of flies over time, or, and additional effect of the mosquitoes was registered in our case. In another study, in free pasturing cattle, the mean additional weight gain in insecticide treated animals was of 8.2 Kg per month, which represent 41 Kg for 5 months (Bruce and Decker 1951a, Bruce and Decker 1951b); results, again, very close to ours.
The present study was designed to measure the impact of hematophagous flies on cattle. However, due to the fly-proof system set-up, our device automatically included a protection against mosquitoes, and the impact measured on cattle kept in open-air also included the effects of flies and mosquitoes on cattle. Evaluating the potential relative role of mosquitoes on cattle, based on the total numbers and rates of diurnal versus nocturnal tail-pedometer records in Experiment 2, the cattle behavior was seriously impacted by insects at night, to which could be attributed, on average, 3,545 beats/night, representing 40% of the additional beats recorded per 24 hours in exposed animals, versus control (Table 6). Whether there is a direct link between the number of tail flicks (or TPRs) and the loss of LBWG is a difficult point to conclude on. Responsibility of the losses measured in this study may be shared between hematophagous flies day-time and mosquitoes night-time, but the relative impact of flies and mosquitoes would be difficult to establish, unless a specific experimental study be designed for that purpose. Most of the authors use to consider mosquitoes as negligible pests for livestock, as said by Hill : « The feeding adults will take some blood and cause some irritation, but not usually a great deal » (Hill 1997). In fact, annoyance and abundance of mosquitoes are certainly under-estimated because they occur at night, after the end of working-hours. The potential impact of mosquitoes on livestock should be reconsidered in the light of our results. As an example, in dairy cattle farms in Thailand, where farmer’s habitations are very close to cattle pens and stable, animals’ owners very often complain about huge mosquito annoyance for themselves and their cattle. A topic on which limited investigations were carried out so far. On that aspect, the use of tail pedometers proved to be very useful, by providing continuous nocturnal and diurnal data that can easily be split to analyse relative diurnal and nocturnal pest attacks.
4.2 Conclusion
These experiments allowed to demonstrate the highly significant impact of flying insects on cattle, including: (i) a high loss of energy evidenced by tail flicks pedometer-records, showing, day-time, 12 times more tail flicks in cattle exposed to flies, (ii) a red cells depletion evidenced by a significantly decreased PCV in exposed animals, 3–4% below the control group, (iii) a decreased cost-benefit on the feed, due to significantly increased by 33% of feed conversion ratio (FCR) in exposed cattle, (iv) a significant loss averaging 10–11% of LBWG in KPS beef feeder cattle 10–15 month of age, accounting for a money loss of $160 within 5 months, and (v) a non-negligible potential impact of mosquitoes on cattle, evidenced by an important increase of tail flick defense movements at night (averaging 22 times more TPRs than non-exposed animals), requiring further evaluation.
Overall the 5-month study in Experiment 2, diurnal TPRs represented 60% of the total TPRs in exposed animals, suggesting a predominant impact of hematophagous flies on cattle, however, leaving room for a potential role of mosquitoes in total losses. The total loss of LBWG registered in this study was equivalent to 10–11% of the cattle LBW, lost within a season of insect activity; a part of this money could fruitfully be invested in fly control, providing control methods are safe and cost-efficient.