Effects of Temperature and Relative Humidity on the Embryonic Development of Hypera Postica Gyllenhal (Col.: Curculionidae)
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insects Article Effects of Temperature and Relative Humidity on the Embryonic Development of Hypera postica Gyllenhal (Col.: Curculionidae) Alexandre Levi-Mourao 1 , Filipe Madeira 2 , Roberto Meseguer 1 , Addy García 1 and Xavier Pons 1,* 1 Department of Crop Protection and Forest Sciences, University of Lleida, 25198 Lleida, Spain; [email protected] (A.L.-M.); [email protected] (R.M.); [email protected] (A.G.) 2 MORE CoLab. Mountains of Research Collaborative Laboratory, Environmental and Ecosystem Management, 5300-358 Bragança, Portugal; [email protected] * Correspondence: [email protected] Simple Summary: The alfalfa weevil, Hypera postica, is a destructive pest around the world and has become, in recent years, the most important pest of alfalfa in Spain. The damage from these weevils reduces the alfalfa forage yield and quality. The factors that affect the population development are poorly known in Europe. This study integrates laboratory experiments and field sampling to address population growth at different temperature and humidity conditions. In the laboratory experiments, we evaluated the combined effect of eight temperatures (8, 12, 16, 20, 24, 28, 32, and 36 ◦C) and three relative humidity regimes (low, medium, and high) on the survival and development time of eggs of Spanish pest populations. The lowest egg survival occurred at high temperatures and low relative ◦ humidity. The egg developmental time decreased from 8 to 32 C, and, at low relative humidity (RH), this time was longer. The minimum threshold and thermal requirements for egg development Citation: Levi-Mourao, A.; were determined. In the field sampling, we obtained information regarding the occurrence of eggs Madeira, F.; Meseguer, R.; García, A.; and larvae (in winter) and the adult reproductive status (in autumn). Combining the laboratory and Pons, X. Effects of Temperature and field results, the oviposition period was determined. The results of the study contribute to a better Relative Humidity on the Embryonic understanding of the annual cycle of H. postica in Spain and Europe and are useful for developing Development of Hypera postica sustainable pest management strategies. Gyllenhal (Col.: Curculionidae). Insects 2021, 12, 250. https:// Abstract: The combined effect of the temperature and relative humidity on the survival and develop- doi.org/10.3390/insects12030250 ment time of the eggs of Spanish populations of the weevil Hypera postica, a key pest of alfalfa around Academic Editor: Mariana F. Wolfner the world, was evaluated under laboratory conditions. The experimental temperatures ranged from 8 to 36 ◦C, in 4 ◦C increments. Three relative humidity ranges were defined: high, medium, and low. Received: 8 February 2021 Eggs of the alfalfa weevil successfully developed until larval emergence at all of the 24 conditions Accepted: 13 March 2021 tested. However, the temperature and relative humidity affected the survival of the eggs. The egg Published: 16 March 2021 developmental time decreased as the temperature increased from 8 to 32 ◦C, and the longest time was recorded at a low relative humidity (RH). The relationship between the development rate and Publisher’s Note: MDPI stays neutral temperature fit well to the lineal model for relative humidity. The minimum development threshold with regard to jurisdictional claims in ◦ (T0) and the thermal requirement for egg development (K) ranged between 3 and 4 C and 209 and published maps and institutional affil- 246 degree-days, respectively. According to these values and the occurrence of eggs and larvae (in iations. winter) and adults (in autumn) in field samplings, the period of oviposition was determined. The results of the study contribute to better understanding the annual cycle and phenology of H. postica in the Iberian Peninsula and southern Europe. Copyright: © 2021 by the authors. Keywords: alfalfa weevil; egg; survival; rate of development; oviposition period; phenology Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons 1. Introduction Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ The alfalfa weevil, Hypera postica (Gyllenhal) (Coleoptera: Curculionidae) is a very 4.0/). destructive pest of alfalfa, Medicago sativa L., which is the world’s most valuable cultivated Insects 2021, 12, 250. https://doi.org/10.3390/insects12030250 https://www.mdpi.com/journal/insects Insects 2021, 12, 250 2 of 13 forage crop. According to the European literature, females lay eggs in clusters inside the alfalfa stems at the end of autumn and winter [1]. The damage, produced by larvae, consists of defoliation reducing the yield and quality of the forage and causing economic losses in many regions of the world [2–4], including the Ebro Valley (north-eastern Iberian Peninsula) [5], where 15–20% of the alfalfa in the Mediterranean region is cultivated. The alfalfa weevil has increased its pest status in the last years and is currently the k-pest of alfalfa in this region [6]. However, the reasons for this increase are still unknown. Although a potential development of insecticide resistance has been suggested [7], this has not been demonstrated, and other factors, such as crop management or environmental conditions, could contribute. The present study is focused on this last aspect. Despite its importance as an alfalfa pest, the information on the biology, life cycle, and ecology and, therefore, the understanding on how the abiotic environment may affect the alfalfa weevil in Europe is scarce. Temperature is a critical abiotic factor that directly influences the insect life processes of survival, development, reproduction, and biological activities and, hence, their population dynamics [8,9]. Temperature is likely the most important environmental variable affecting embryonic development [10]. The eggs of most coleopteran species develop within a range of temperatures with upper and lower thresholds, outside of which, development is retarded or inhibited [11–14]. The available environmental humidity is another important requirement for egg development [15]. The eggs of some species can obtain sufficient water from the moisture of the air, whereas those of other species must be in contact with water for development [10]. Eggs kept under too dry of conditions may fail to hatch: in some cases, because the embryo within is desiccated; in others, because the chorion itself becomes too hard for the young insect to escape [15]. In many coleopteran insects, egg development is delayed by low humidity in the surrounding air, presumably due to the lack of water in it [16]. However, temperature and water availability may interact, modulating their single effects [17]. Most of the studies regarding the temperature-dependent development of H. postica have been focused in larvae and pupae and have been carried out with populations from North America and Middle East Asia [14,18]. The results of these studies suggested wide variations in geographical regions [19–21]. On the other hand, literature dealing with the effect of abiotic factors on embryonic development is scarce. How successfully and quickly eggs develop will determine the start of the damage period and the quantity of damage. A period of warm weather during the beginning of oviposition results in a very large number of eggs laid in a short time. However, if the temperatures are mild, the laying period can be extended for much of autumn and winter [22,23]. Within the framework of a wider study targeted to determine the effect of the en- vironmental abiotic factors on the development of H. postica populations and to predict the occurrence and phenology in southern Europe, the present study aimed to evalu- ate the combined effects of the temperature and relative humidity on the survival and developmental time of eggs. To do that, we submitted egg batches to different constant tem- peratures and to three ranges of relative humidity. Field samplings were also performed to record the occurrence of adults in autumn and the winter egg phenology to determine the reproductive adult period and validate the predictions made from the laboratory results. 2. Materials and Methods 2.1. Laboratory Tests Constant temperatures were established and maintained in separated cabinets with an 8:16 L:D photoperiod. The experimental temperatures ranged from 8 to 36 ◦C, in 4 ◦C increments. Three relative humidity (RH) ranges of the air in the experimental rearing cages were defined: high: 90% < RH < 100%; medium: 50% < RH < 75%; and low: 10% < RH < 35%. The predetermined RH was maintained by the use of agar-agar in high range; with only the biological material covered with mousseline in the medium range; and silica gel 2.5–6 mm with indicator (without cobalt chloride) PA-ACS (PANREAC Barcelona, Spain) in the low range. The temperature and RH were measured using a HOBO® Pro Insects 2021, 12, 250 3 of 13 v2 logger U23-00x (ONSET, Cape Cod Massachusetts, MA, USA) probe at the level of the rearing boxes and recorded every 10 min during the experimental period. Eggs from H. postica were obtained by rearing North-East Spain field-collected adults in 2000 mL glass jars, covered with mousseline for proper ventilation. For egg laying, fresh alfalfa stems were provided on a daily basis. Stems were placed into a filled water glass vial sealed with parafilm to prevent dehydration. Then, the 24 h-old stems were removed and dissected for egg counting. Therefore, the experiment was set with eggs laid in the preceding 24 h. Stems with clusters of 10–15 eggs were placed in transparent PVC rearing cages (53 mm diameter to 32 mm high), and 20 replications for every temperature/RH combination were established. The cages were examined at daily intervals, and the number of living eggs and egg colour (distinguishing between yellow, green-brown, and head capsule visible or black (adapted from [23]) was recorded until hatching. As the number of eggs in each cluster varied, the proportion of the finally hatched eggs related to the initial number of eggs was calculated for each replication.