Introduction

Opuntia ficus-indica (L.) Mill. (Cactaceae), known as prickly pear cactus, is a multipurpose plant of great significance due to its ability to tolerate high temperatures, dry conditions, and marginal areas (^{Barbera et al., 1992}). ^{Stintzing and Carle (2005)} characterized the prickly pear cactus as a "miracle plant, dromedary of the plant world and bank of life"; a plant of great agricultural importance in the semi-arid area of Tigray, Ethiopia (^{Gebremeskel et al., 2013}). Almost all rural communities in eastern and southern Tigray and some parts of central Tigray use cacti as a staple food for about four months and as a cash crop, covering 360,000 ha of land, of which half is planted around home yards (^{Brutsch, 1997}).

The cochineal insect, Dactylopis coccus Costa (Hemiptera: Coccomorpha: Dactylopiidae) is a prickly pear cactus insect that reproduces sexually; the female has 2 nymphal instars before reaching the adult stage, whereas the males go through 2 feeding nymphal instars, and non-feeding prepupal and pupal stages before reaching the adult stage which has a pair of wings (^{Pacheco da Silva et al., 2022}). Dactylopius spp. is often considered beneficial for producing a red dye and also used for controlling weedy cacti (especially in South Africa and Australia). However, they are considered pests when they attack cultivated cacti (^{Kondo, 2022}). Dactylopius coccus has been reared commercially as a source of cochineal dye for centuries and is widely used as a coloring agent in foods, including beverages, cosmetics, and pharmaceutical products, and to dye textiles; the dried body of adult females (known as ‘grana’) is produced commercially in Bolivia, Chile, Mexico, Peru and Spain (^{Pacheco da Silva et al., 2022}). Damage caused by D. coccus and other species of the genus Dactylopius includes chlorosis (yellowing of the cladodes), dehydration and weakening of the host, and dieback of the plants in 6-10 months (^{Pacheco da Silva et al., 2022}). On several continents, Dactylopius species pose a threat to prickly pear cactus. D. coccus was reported to damage large areas planted with prickly pear cactus in Tigray in Ethiopia (^{Belay, 2015}; ^{Berhe et al., 2020}), and D. opuntiae is a significant pest that has been rapidly spreading over several nations, especially in the Mediterranean basin (^{Bouharroud et al., 2016}; ^{Kondo, 2022}; ^{Mazzeo et al., 2019}). Prickly pear cactus production suffers severe losses because of the D. opuntiae epidemic in Brazil's semiarid region (^{Pacheco da Silva et al., 2022}; ^{Torres & Giorgi, 2018}). The pest consequences of D. coccus in Tigray, Ethiopia, are socio-economic losses such as income reduction and youth migrations (^{Berhe & Siyum, 2022}). It is also registered as a pest in Chile (^{IPPC, 2010}).

Developing and utilizing resistant varieties is a vital constituent of an integrated pest management strategy to control D. coccus (^{Berhe et al., 2020}; ^{Primo, 2020}; ^{Sabbahi & Hock, 2022}). ^{Tovar et al. (2005}) reported that D. coccus performance was poor in certain varieties. Since D. coccus had not established itself, ^{Méndez-Gallegos et al. (2010)} suspected some form of resistance. Hence, selecting cochineal-resistant prickly pear cactus genotypes is important for mitigating the pest problem (^{Berhe et al., 2020}).

Materials and methods

The experiment was conducted in a greenhouse, at the Center of Biology and Agriculture Sciences (CUCBA) of Guadalajara University in Mexico from December 2021 to April 2022 in a greenhouse. Four genotypes were tested: ‘Sandate 2, ‘Punto 1’, and ‘Punto 2’; and one susceptible host (‘Chicomostoc’). These genotypes and their species were selected based on field observations and information obtained from growers. The experiment was arranged in a completely randomized design (CRD) with three replications (cladodes as an experimental unit). The plants were grown in pots and cladodes for each plant were selected to be inoculated, while they were attached to their mother plant. ‘Punto 1’, and ‘Punto 2’ were grown from seeds while the others were vegetatively propagated. Adult females of D. coccus were reared in separate cladodes and were prepared for inoculation. The adult females were removed from their original host cladodes with a fine brush and kept in small paper bags. The insects were inoculated by fixing the paper bags with spines holding 20 mature females each. The paper bags remained attached to the cladodes for seven days allowing the ovipositing and infestation of the cactus pear cladodes (Figure 1a-d) (^{Gusqui Mata, 2013}; ^{Vigueras et al., 2005}).

The number of first-instar nymphs after one week of infestation, and the number of adult females at maturity (100 days after infestation), were counted. The number of first-instar nymphs was counted seven days after infestation, while the number of adult females was counted when they started oviposition. The data were subjected to descriptive statistical analysis and average values were calculated.

Results

All genotypes had first-instar nymphs (crawlers) on the 7^th day of infestation with a number ranging from 180 to 350 (Table 1 and Figure 1 a-d). The nymphs reached maturity at the host genotype (‘Chicomostoc’) and 224 individuals were counted (Table 1 and Figure 1 f-h), remained at the first-instar stage at ‘Punto 1’, the whole life cycle period, but died at ‘Punto 2’ and ‘Sandate 2’ genotypes (Figures 1b and c). Similar screening works for Dactylopius spp. resistance has been reported before. ^{Kuti (1990}) identified five resistant Opuntia genotypes, while ^{Batista et al. (2022)} screened four D. opuntia genotypes as resistant out of 121 tested genotypes. ^{Berhe et al. (2022)} also found one resistant O. ficus-indica cultivar against D. coccus. The genotypes may be resistant through several mechanisms/strategies, both chemical and physical (^{Akroud et al., 2021}). Being in the first-instar stage at accession ‘Punto 1’ is an indication of Opuntia’s variable resistance mechanisms (^{Musengi et al., 2021}).

Figure 1 Cochineal insect infestation status of Opuntia ficus-indica genotypes tested after 7 days of inoculation, white spots are observed that show the presence of cochineal insects (a, Chicomostoc; b, Punto 1; c, Punto 2; and d Sandate 2), and after the completion of the insect life cycle period (e, Chicomostoc, a susceptible host with cochineal insects that completed the life cycle; f, Punto 1 (resistant) the insects could not develop beyond the first-instar stage; g, Punto 2 (resistant) without cochineal insects; and h, Sandate 2 (resistant) without cochineal insects. 

Conclusion and recommendations

The evaluated genotypes of O. ficus-indica cultivars, namely ‘Chicomostoc’, ‘Punto 1’, ‘Punto 2’, and ‘Sandate 2’ showed different responses to D. coccus. All genotypes except ‘Chicomostoc’ were found resistant to D. coccus because the insects could not develop and complete their life cycle, and died at the first-instar stage, except on ‘Punto 1’ at which the insects remained at the first-instar stage. These resistant genotypes can be used as part of D. coccus integrated management in areas where this insect is considered a pest. They can also be used for genetic improvements and other research purposes. The chemical and physical characteristics of the genotypes related to D. coccus resistance mechanisms are an important subject of study.