**5. Insecticide resistance in insect eggs**

Insect eggs, like other insect stages, vary in their susceptibility to insecticides. Susceptibility differences between eggs from different insect species may be due to variability in chorionic adaptations that facilitate the uptake of oxygen [10]. Fewer aeropyles or smaller aeropyles may reduce the amount of insecticide that can enter the egg. In addition to respiratory structures, insecticides may also enter the chorion through the micropyles that allow fertilization of the egg [25]. Furthermore, modifications of the chorionic structures of the insect eggshell may enhance or reduce the penetration of insecticides.

Another important consideration when determining susceptibility of eggs to insecticides is the age of the egg. A freshly laid egg that has not fully developed is usually more susceptible to insecticides compared to an egg that has aged several days. Insect egg susceptibility to insecticides also changes during embryonic development [9]. The eggshell itself may harden during development, and the embryo may produce enzymes that break down insecticides.

Few studies have focused on insecticide resistance in eggs. Insecticide resistance in eggs from different insect species has been demonstrated when resistance was already quantified in the adult or larval stages [26–30]. Eggs collected from insect strains with resistance in the adult stage have been shown to have similar resistance. These studies determined that insecticide resistance had developed in eggs but did not determine the type or mechanisms of resistance.

Bed bugs, *Cimex lectularius*, have been extensively studied as adults with regards to insecticide resistance. Molecular technology has revealed that adult bed bugs have three types of resistance to pyrethroid insecticides: enhanced enzyme detoxification, KDR resistance, and target-site insensitivity. Recent research has revealed that bed bug eggs are also highly resistant to pyrethroids (Figure 4). Eggs collected from a strain that is considered pyrethroid-susceptible (Harlan) were much easier to kill with the insecticide deltamethrin, compared to eggs collected from strains (Richmond and Epic Center) where pyrethroid resistance was previously determined in adults.

several cases. Eggs of the greenhouse whitefly, *Trialeurodes vaporariorum*, were found to be nonsusceptible to the fungus *Aschersonia aleyrodis* [23]. No fungal spores or discoloration of the eggs was observed, and there were no differences in mortality between eggs that were treated with the fungus compared to eggs that had not been treated. However, the fungal spores were persistent for several days, so when the first instar larvae emerged from the egg,

Five different fungi, (*Beauveria bassiana, Metarhizium anisopliae, Metarhizium flavoviride, Paeci‐ lomyces farinosus*, and *Paecilomyces fumosoroseus*) were tested against eggs of the curculionidae beetles, *Otiorhynchus sulcatus* and *Sitona lineatus* [24]. *S. lineatus* eggs were much more tolerant of the fungal pathogens compared to *O. sulcatus* eggs. *O. sulcatus* eggs were found to be susceptible to all fungi except for *B. bassiana*. Only one fungus, *M. flavoviride*, was found to be moderately effective against *S. lineatus* eggs, causing 32% egg mortality. No other fungal

Insect eggs, like other insect stages, vary in their susceptibility to insecticides. Susceptibility differences between eggs from different insect species may be due to variability in chorionic adaptations that facilitate the uptake of oxygen [10]. Fewer aeropyles or smaller aeropyles may reduce the amount of insecticide that can enter the egg. In addition to respiratory structures, insecticides may also enter the chorion through the micropyles that allow fertilization of the egg [25]. Furthermore, modifications of the chorionic structures of the insect eggshell may

Another important consideration when determining susceptibility of eggs to insecticides is the age of the egg. A freshly laid egg that has not fully developed is usually more susceptible to insecticides compared to an egg that has aged several days. Insect egg susceptibility to insecticides also changes during embryonic development [9]. The eggshell itself may harden during development, and the embryo may produce enzymes that break down insecticides. Few studies have focused on insecticide resistance in eggs. Insecticide resistance in eggs from different insect species has been demonstrated when resistance was already quantified in the adult or larval stages [26–30]. Eggs collected from insect strains with resistance in the adult stage have been shown to have similar resistance. These studies determined that insecticide resistance had developed in eggs but did not determine the type or mechanisms of resistance. Bed bugs, *Cimex lectularius*, have been extensively studied as adults with regards to insecticide resistance. Molecular technology has revealed that adult bed bugs have three types of resistance to pyrethroid insecticides: enhanced enzyme detoxification, KDR resistance, and target-site insensitivity. Recent research has revealed that bed bug eggs are also highly resistant to pyrethroids (Figure 4). Eggs collected from a strain that is considered pyrethroid-susceptible (Harlan) were much easier to kill with the insecticide deltamethrin, compared to eggs collected from strains (Richmond and Epic Center) where pyrethroid resistance was previously

they became infected with the fungus.

92 Insecticides Resistance

treatments resulted in egg mortality to *S. lineatus* eggs.

**5. Insecticide resistance in insect eggs**

enhance or reduce the penetration of insecticides.

determined in adults.

**Figure 4.** Mean percent mortality of eggs from a susceptible strain of bed bug eggs (Harlan) and two pyrethroid-resist‐ ant strains (Richmond and Epic Center) that were treated with the insecticide deltamethrin (0.05%). Bed bug eggs from the resistant strains had much lower mortality values compared to the susceptible strain eggs at each tested rate, show‐ ing that resistant eggs were much more difficult to kill. This study determined that resistance had already developed in the egg stage of bed bugs (Taken from Campbell et al. 2015 [30]).

Head lice, *Pediculus humanus capitis* (Phthiraptera: Pediculidae), have been shown to be highly resistant to pyrethroid insecticides [26]. Eggs, nymphs, and adults were evaluated from three different resistant head louse populations. Eggs were found to be highly resistant to perme‐ thrin in populations that had already demonstrated a high resistance to pyrethroid insecticides in adults and nymphs. This study suggests that there are similar resistance mechanisms within head louse eggs and adults from the same population.

Resistance patterns between eggs and first instars of Reduviid bugs, *T. infestans*, have been evaluated [31]. Insecticide resistance varied between eggs from different populations of *T. infestans* that were collected throughout Argentina and Bolivia. Eggs from a resistant strain that were aged several days were found to be as resistant to deltamethrin as the first instars (Toloza et al. 2008). Also, eggs from the resistant strain were found to be resistant to lambdacyhalothrin but susceptible to fipronil and fenitrothion. First instar nymphs from a resistant strain had similar patterns of resistance as the eggs.

Eggs have also been found to be more tolerant than adult stages of the lesser grain borer, *R. dominica,* to the fumigant phosphine. Not only were eggs more tolerant to phosphine, eggs that were collected from adults that were previously screened for resistance were harder to kill with phosphine compared to eggs collected from strains that were previously determined to be susceptible to phosphine [27]. Screening for resistance in the adult stage against the fumigant phosphine was a reliable indicator for determining resistance in the egg stage of the lesser grain borer, with 9 of the 10 strains of adults that were phosphine resistant also exhibiting resistance in the egg stage. Eggs of *Liposcelis bostrychophila*, a stored product pest, were found to have a delay in embryonic development when fumigated with phosphine. This delay in embryonic development seems to be a method of resistance, causing control failures with phosphine treatments because the eggs were able to survive treatment this creates a problem for grain storage facilities because they may later find a reinfestation of the pest after the eggs hatch.

There is a lot left to learn about insect eggs and their resistance to different control methods. The lack of knowledge on egg biology, physiology, and control compared to other life stages is unfortunate when you consider how important this life stage is in relation to potential management strategies. Most studies that have evaluated the efficacy of insect control methods have mostly neglected the egg stage and have focused on adult or immature stages.

When the egg stage is ignored during the implementation of treatments, those eggs are left to hatch and possibly cause a reinfestation. Therefore, more studies on the efficacy of control treatments against eggs are needed, especially in cases in which the eggs are reasonably accessible and treatable. Insect eggs should not be ignored in pest management programs just because they are small, or do not bite or feed. Rather, control efforts targeting insect eggs are advantageous because the pests would be eliminated before it has a chance to cause any damage.
