Biological Cycle of Bug Pseudotheraptus devastans Distant (Heteroptera: Coreidae), the Coconut Pest in Côte d’Ivoire

Pseudotheraptus devastans Distant (Heteroptera: Coreidae), is an important coconut pest in Côte d’Ivoire. The stings from Nymph and adult cause not only the falling of immature nuts, but also their deformation. This triggers losses of production in the order can reach 80%. Little information is available about this pest. Those existing are limited to its ecology and damages caused in host-plants. The present investigation is aimed to study the life cycle of this pest. The experiment was carried out in a coconut field of the Marc Delorme Coconut Research Station (MDCRS). On these coconut palms, bunches between 3 and 4 months of age, enclosed in cage were used. Development duration from an egg to adult emergence averaged 28.26 days. The average incubation period was on average 6.63 days and fertility rate of eggs was 98.01%. Nymphal development period was 21.63 days. The sex-ratio is in favour of females which live 112.6 ± 13.45 days. These females laid on average 198.85 ± 36.05 eggs. The males lived on average 131.3 ± 17.7 days. The knowledge of these different parameters could allow the development of a control method based on the breaking of life cycle of this pest.


Introduction
The bug Pseudotheraptus devastans Distant (1917) belongs to the Class Insecta, Order Hemiptera, Suborder * Corresponding author. Heteroptera, Family Coreidae, Genus Pseudotheraptus and Species devastans [1]. This bug is a serious coconut pest (Cocos nucifera L.) in Côte d'Ivoire. The yielding losses can reach 80% [2] [3]. Nymphs and adults attack the youngest nuts under 7 months old. Their stings trigger the premature falling of the young nuts aged from 2 to 3 months [4] or severe deformations of the future old nuts. Two reasons explain this premature falling. Firstly, during feeding, the stylet caused damages to tissues of the plant. Secondly, the saliva of bug contains toxin which have lytic action on the cellular components of coconut [5] [6]. P. devastans have been specifically implicated as virus vectors. However, penetration of the stylet into plant tissue facilitates entry of various other pathogenic microorganisms such as fungi [7]. P. devastans feeding lesions have been associated transmission of the fungal diseases of cassava (anthracnose cassava) [5] [8], (candlestick disease) [9]. In light of the damage caused, several initiatives of control were proposed. The chemical control, although efficient, can be very harmful. Indeed, human health and environmental balance can be affected. Likewise, insects can develop resistances [10] [11]. Studies geared towards identification of ecologically healthy methods may prove more effective. Nonetheless, they require the knowledge more detailed of the bug biology. Almost all of the information on Pseudotheraptus devastans, results from works led in Côte d'Ivoire and in Republic of the Congo on the pest ecology and damages caused to the host plants [2] [4] [5] [9]. The work carried out aimed to study the biological cycle of P. devastans for further control purposes.

Location
The study was conducted on station at the Coconut Research Station of CNRA (National Center of Agronomic Research) is located at Port-Bouët (5˚15'N and 3˚55'W) in Abidjan in the southern part of Côte d'Ivoire. The International Coconut Genebank for Africa and Indian Ocean is hosted by this station. The studies in the field took place from January to December, 2014 at an average temperature and a relative humidity of 26.39˚C and 87.85% respectively with 2078 mm annual rainfall.

Mass Rearing
The first operation consisted to collect nymphs of Pseudotheraptus devastans from infested coconut bunches. Collected nymphs are putting on new healthy bunches with nut aged from 3 to 4 months in trapezoid cages measuring 65 × 48 × 33 cm. the cage covered all the nut. These nymphs were monitored until adults' emergence.

Development Cycle and Some Biological Parameters of Pseudotheraptus devastans
Twenty pairs (females and males) of newly emerged adults are formed and put each in a new cage on a new coconut bunch. These cages are daily observed. Every egg laid was dated, marked and observed until its hatching. The date of hatching was recorded. At hatching, newly emerged first instar reared two by two in a cage containing a new old coconut bunch aged from 3 to 4 months for their food until adults' emergence. To facilitate the location of nymphs in the course of observations, bunches must contained spaced nuts. Each nymph was daily monitored for molting. This was confirmed by the presence of exuviae at successive stages. From the first laying, the number of eggs laid by each female was scored until the death of this female. The total number of eggs laid by female was counted and the mean of laid eggs was calculated. The following parameters were determined (the duration of adults life, the duration of incubation, the duration of nymphal stages): -the duration of incubation is the period stretching out from the laying to hatching of eggs.
the duration of nymphal stages is the period between one instar and the next instar.
the duration of adults life is the time between emergence date and death date.

Data Analysis
Means and standard deviation related to mean were computed for the durations of the different life stages. These were, namely eggs incubation, different nymph instars, adult survival age as well as and fecundity such as the number of eggs laid by each female in its lifetime. The survival of nymph instars calculated as the percentage of individuals at a specific stage that progress to the next stage. In the same way, the fertility rate and sex ratio, namely the proportion of female adults, were computed.

Life Cycle
Detailed description of all stages is provided by [2]. Indeed, Adult are coloured brown-red except on the ventral face of abdomen where it is pale green, the membrane of the hind wings is black and the antennae are filiforme and almost long as the body. The nymph appearance and colour is similar to those of the adult. The antennae are long, with 2 and 3 flattened. Depending on the instar, there are 2 to 4 blackish-brown, rounded protuberances on the dorsal face of the abdomen. Eggs are lentiform and their colour changes from pale yellow to red-brown with age.
In the present study, life cycle of Pseudotheraptus devastans Distant (1917) was analyzed and it showed three stages of development, namely the egg, nymph and adult. Observations showed that it's a hemimetabolous development as 89 families component the suborder of Heteroptera [12]. Indeed from the hatching until adult emergence, we observed successively 5 exuviaes, consequences in the various sloughs, which are the proof of the passage from a stage to another (Figure 1).
The duration of the first instar was the shortest while that of the last instar was the longest. This fact is also observed at several coreids species such as Mozena obtusa [21], C. scutellaris [20], Corecoris dentiventris [22], P. wayi [14] [17]. Anatomical, morphological and physiological transformations undergo and acquire by fifth instar before adult stage would explain the longer duration of this instar.
According to [22], the differentiation of the internal reproductive organs and the completion of the wings development occurs at this instar. The average cumulative nymphal development time in days was 21.63 ± 1.47. The total duration of the immature stages of P. devastans (21.63 ± 1.47 days) is close to that obtained by [23] who reported that the immature stages of Coreidae border on 21 days. Total developmental duration (egg to adult emergence) averaged 28.26 ± 1.55 days (Figure 2).
This duration is included in the same time interval (21 -40 days) by the nearby species P. wayi [14]. The instars survival rates during rearing were 83.33%, 64.61%, 90.47%, 78.94%, 86.66% respectively (Figure 3). The   Lowest nymphal survival occurred during the second instar. This observation seems to be the rule to several coreids [21] [22] [24] and has to be a critical feeding phase. It could be explained considering that in this instar actual feeding activity begins, implying in the need to metabolize for the first time toxic or non-nutritive compounds (allelochemics) contained in the food.

Conclusion
This study has allowed knowing duration of all development stages of the life cycle of P. devastans. Three main steps were observed during life cycle of Pseudotheraptus devastans. These steps are: egg stage, nymph stage characterized by five slougs and adult stage. This shows that P. devastans is hemimetabolous. The development duration from an egg to adult emergence averaged 28.26 ± 1.55 days, with an incubation period of 6.63 days and nymphal development period of 21.63 days. Males with 131.3 ± 17.7 days live longer than females with 112.6 ± 13.45 days. The high eggs-laying (198.85 ± 36.05 eggs), the very high rate of egg viability (98.01%) and the sex-ratio in favour of the females are potential of reproduction indicators of this species. The acquisition of all these elements added to the fact that all the stages of development except eggs, are infesting, require news approaches and the implementations of methods of more adequate and more efficient control.