Dermanyssus gallinae

Soulié, Anne-Sophie, Sleeckx, Nathalie & Review, Lise Roy, 2021, Repellent properties of natural substances against Dermanyssus gallinae: review of knowledge and prospects for Integrated Pest Management, Acarologia 61 (1), pp. 3-19 : 11-13

6 Framework on the integration of the repellent properties of substances of plant origin in Integrated Pest Management strategies for D. gallinae

Generally, the effect of repellent substances on a population of D. gallinae may be the result of different factors, depending on the type of repellency, the time in the life cycle of the pest and the mode of application of repellents in henhouses. True repellents can be used to develop methods that are based on remote interference. According to a mathematical model, an increase in the time between blood meals in female mosquitoes can have a significant effect on the population dynamics of this blood­feeding insect ( Wan et al. 2014). If this also applies to D. gallinae , it is therefore possible to hypothesize that making the hen repellent or less attractive via plant­based feed supplementation (e.g., El Adouzi et al. 2019) could contribute to delay the population growth of D. gallinae by extending the duration of fasting between two blood meals and thus lengthening the time between laying or molting. Another hypothesis would be that pushing D. gallinae away from its microhabitats by means of true repellent substances could reduce the maximum size of infestation in farm buildings by limiting the availability of favourable habitats and exposing mites to increased contact with conventional acaricides with biopesticides. To succeed in this direction, a method should be found to apply the repellent in the microhabitats in question, through local spray applications at the perch level or elsewhere, for example.

Plant­derived true repellents could even be used in combination with attractants to develop

“push­pull” approaches ( Cook et al. 2007). The principle of push­pull involves combining an attractant and a repellent in a synergistic way, attracting pests to a “pull” stimulus whilst simultaneously repelling them with a “push”. In this way, individually moderate attractant and repellent properties can have a synergistic effect. Traps baited with a hen­mimicking attractant,

for example, could attract more mites if deployed in combination with a repellent delivered to live hens (e.g., through feed supplementation) or the surrounding environment. A summary of repellent and attractant volatile compounds active on D. gallinae has recently been provided by Gay et al. (2020) and may help in identifying stimulo­deterrent product combinations for further development of push­pull approaches.

Irritants can be used to form local barriers, for example, to limit the access of mites to perches (to feed) or microhabitats (to rest), though they would need to be applied precisely and in a form that could provide a persistent effect, despite accumulation of dust and debris. They could possibly be used to make the skin of the hen less welcoming to D. gallinae , in the same way that certain products are used against mosquitoes (e.g., DEET, see DeGennaro 2015). Here again, the mode of application needs to be considered: finding a way to make an irritant emit directly from the hen through a per os administration would open up promising possibilities.

Otherwise, direct application to the birds (e.g., by dipping them in a solution) may help to discourage the mite from biting the hen, but it is likely to be impractical at farm level where flock sizes number in the thousands.

Methodologically speaking, to state whether plant­derived repellents may be useful in controlling a pest, it is necessary (1) to conduct in­vitro experiments to state their repellent properties and, if possible, how mite­related and mite­independent factors interfere with them,

(2) based on the obtained results, decide how and where to apply the substance on the farm, and

(3) to verify that the properties measured in vitro have repercussions up to the level of the pest population under field conditions (here, in egg­laying henhouses). A relatively large body of literature from in vitro tests is already available: it is therefore possible, at least for the plants listed in Table 1, to derive data on their repellent properties against D. gallinae . However, for many of these products, further testing would be useful, especially as the type of repellency they exert has not been established. In addition, more work is needed on the different factors that may interfere with volatile compounds and affect the repellent properties of the test substances

(e.g., physiological status of the mites, aggregation pheromone, ambient features). To date,

very little information on these aspects is available for D. gallinae .

When considering field use of repellents it is important to consider both the strengths and limitations of in vivo experiments. A wide variety of factors, including the above ambient factors and others (e.g., seasonal variation and mite disease), can vary in the field regardless of the factor tested (e.g., repellency of a natural substance) and interfere with the experiment

(unlike in vitro experiments, which are conducted under controlled conditions). The size of the pest population at the beginning of the experiment is generally unknown and heterogeneous between and within buildings or compartments (unless working on mite­free buildings and deliberately contaminating them with counted mites). Therefore, it is generally impossible to have a true control henhouse or control compartment. The high number of uncontrolled factors makes it difficult to carry out true replicates to distinguish the natural variation induced by the controlled factors from other factors. The population size of D. gallinae in the building is usually estimated by various sampling methods (generally by passive trapping; see Mul et al. 2015 for review) and compared before and after treatment. However, the pre/post treatment comparison does not eliminate uncontrolled concomitant factors that may have an impact on the estimated effectiveness of the treatment. It is therefore difficult to estimate the extent to which the observed results were actually induced by the treatment tested. Finally, the criterion chosen for evaluating the effect on D. gallinae is generally the change in the number of individuals trapped over time. This aims to evaluate the evolution of the mite population size on the basis of sampling (taking into account the performance and limitations of the sampling method used). However, regardless of the sampling method used, since D. gallinae is an animal with a typically highly aggregated spatial distribution, it is very difficult to obtain a good representation of the level of infestation of the building through the trapped population (large disparities in the values obtained per trapping point are expected; thus, a very large number of traps is needed to obtain a good picture). Criteria related to production can be taken into account (e.g., laying rate and feed conversion), but their evolution over time is often too slow to capture the effects of moderate­impact treatments. Physiological indicators from the hens may also be taken into account, as it has been shown that the blood parameters, such as hematocrit or plasma corticosterone levels, are affected by mite infestations both in the wild ( Potti et al.

1999) and on farms ( Kowalski and Sokół 2009). On the whole, as trade­offs between rigor and feasibility must be made, it is then crucial to consider the results of field trials in the light of the experimental strengths and weaknesses to draw rational conclusions: levels of comparability between modalities with and without treatment (true controls available or not, confounding factors in case of before/after design) and natural variability estimated or not (presence or not of true replicates).

In short, by allowing the repellent properties and the mode of application of plantoriginating repellents to control D. gallinae to be confronted, field studies are of undeniable importance. However, their realization requires careful consideration. Performing a sensible experimentation requires that the repellent properties (demonstrated in in­vitro tests), the targeted mode of action at the henhouse level, and the completeness and complexity of the farming system are taken into account.